Geert J Verhoeven | University of Vienna (original) (raw)

Articles by Geert J Verhoeven

Graffiti, by their very nature, are ephemeral, sometimes even vanishing before creators finish th... more Graffiti, by their very nature, are ephemeral, sometimes even vanishing before creators finish them. This transience is part of graffiti's allure yet signifies the continuous loss of this often disputed form of cultural heritage. To counteract this, graffiti documentation efforts have steadily increased over the past decade. One of the primary challenges in any documentation endeavour is identifying and recording new creations. Image-based change detection can greatly help in this process, effectuating more comprehensive documentation, less biased digital safeguarding and improved understanding of graffiti. This paper introduces a novel and largely automated image-based graffiti change detection method. The methodology uses an incremental structure-from-motion approach and synthetic cameras to generate co-registered graffiti images from different areas. These synthetic images are fed into a hybrid change detection pipeline combining a new pixel-based change detection method with a feature-based one. The approach was tested on a large and publicly available reference dataset captured along the Donaukanal (Eng. Danube Canal), one of Vienna's graffiti hotspots. With a precision of 87% and a recall of 77%, the results reveal that the proposed change detection workflow can indicate newly added graffiti in a monitored graffiti-scape, thus supporting a more comprehensive graffiti documentation.

Color data are often required for cultural heritage documentation. These data are typically acqui... more Color data are often required for cultural heritage documentation. These data are typically acquired via standard digital cameras since they facilitate a quick and cost-effective way to extract RGB values from photos. However, cameras’ absolute sensor responses are device-dependent and thus not colorimetric. One way to still achieve relatively accurate color data is via camera characterization, a procedure which computes a bespoke RGB-to-XYZ matrix to transform camera-dependent RGB values into the device-independent CIE XYZ color space. This article applies and assesses camera characterization techniques in heritage documentation, particularly graffiti photographed in the academic project INDIGO. To this end, this paper presents COOLPI (COlor Operations Library for Processing Images), a novel Python-based toolbox for colorimetric and spectral work, including white-point-preserving camera characterization from photos captured under diverse, real-world lighting conditions. The results highlight the colorimetric accuracy achievable through COOLPI’s color-processing pipelines, affirming their suitability for heritage documentation.

Admired and despised, created and destroyed, legal and illegal: Contemporary graffiti are polaris... more Admired and despised, created and destroyed, legal and illegal: Contemporary graffiti are polarising, and not everybody agrees to label them as cultural heritage. However, if one is among the steadily increasing number of heritage professionals and academics that value these short-lived creations, their digital documentation can be considered a part of our legacy to future generations. To document the geometric and spectral properties of a graffito, digital photographs seem to be appropriate. This also holds true when documenting an entire graffiti-scape consisting of 1000s of individual creations. However, proper photo-based digital documentation of such an entire scene comes with logistical and technical challenges, certainly if the documentation is considered the basis for further analysis of the heritage assets. One main technical challenge relates to the photographs themselves. Conventional photographs suffer from multiple image distortions and usually lack a uniform scale, which hinders the derivation of dimensions and proportions. In addition, a single graffito photograph often does not reflect the meaning and setting intended by the graffitist, as the creation is frequently shown as an isolated entity without its surrounding environment. In other words, single photographs lack the spatio-temporal context, which is often of major importance in cultural heritage studies. Here, we present AUTOGRAF, an automated and freely-available orthorectification tool which converts conventional graffiti photos into high-resolution, distortion-free, and georeferenced graffiti orthophotomaps, a metric yet visual product. AUTOGRAF was developed in the framework of INDIGO, a graffiti-centred research project. Not only do these georeferenced photos support proper analysis, but they also set the basis for placing the graffiti in their native, albeit virtual, 3D environment. An experiment showed that 95 out of 100 tested graffiti photo sets were successfully orthorectified, highlighting the proposed methodology’s potential to improve and automate one part of contemporary graffiti’s digital preservation.

Heritage, 2022

Investigating deserted medieval castles and villages in remote rural areas paired with a scarcity... more Investigating deserted medieval castles and villages in remote rural areas paired with a scarcity of meaningful written sources is a challenging task that can be significantly enhanced by the use of non-invasive archaeological prospection methods. Furthermore, the interpolation of stratigraphic relationships among maps by analysing paths and field boundaries, as performed by Klaus Schwarz in the 1980s, can also contribute significantly. Thus, in order to resolve numerous unanswered questions, a multidisciplinary approach is required. In this paper we present preliminary archaeological prospection data using magnetics and airborne laser scanning (ALS) as well as methodological considerations on the systematic analysis of historical maps on the site of Dernberg, a medieval motte-and-bailey castle with an adjoining deserted village. The magnetic data and corresponding aerial images, although not providing decisive information on internal structures, show several historical roads that allow for the localisation of the village at the foot of the castle hill, as well as other pathways and defensive structures. Data derived from laser scanning surveys carried out by uncrewed aerial vehicles, respectively, drone flights allow for a significant gain in information compared to publicly available ALS data. In a methodological discussion on the systematic analysis of historical maps, the site of Dernberg is used to illustrate not only how such an analysis can determine chronological sequences with respect to the pattern of former agricultural field systems and road networks, but that these assumptions can be confirmed in part by geophysical surveys.

The Borre Monitoring Project investigated how environmental factors, in particular, precipitation... more The Borre Monitoring Project investigated how environmental factors, in particular, precipitation and soil moisture variation as well as different soil and sediment types, affect the quality of GPR data collected for archaeological purposes. To study these questions, regular GPR surveys were conducted over a period of 14 months across a test area covering a hall building at the Iron and Viking Age site of Borre in Norway. In order to obtain in situ measurements of environmental factors relevant for electromagnetic wave propagation including volumetric water content, bulk electrical conductivity, ground temperature, and precipitation, three monitoring stations were erected at the test site. Soil and sediment samples taken from the profiles at the respective monitoring stations were analysed to gain a basic description of their physical and chemical properties. Twelve GPR surveys were conducted roughly once a month between August 2016 and September 2017 and the results clearly indicated differences in the quality of the data collected. To better understand the underlying causes for this variation, GPR data were compared against and integrated with the in situ measurements gathered using the monitoring stations. The results of this analysis emphasised the benefit of dry conditions, which, if prevailing over a longer period of time, proved to generate GPR data of the highest quality. Seasonality could not be attested; instead, data quality was governed by small-scale weather patterns, where the time and intensity of rainfall events prior to the surveys as well as sudden changes in air temperature played a decisive role. While the results of this study are only valid for sites with similar settings such as Borre, they emphasise the importance of considering the environmental factors during all stages of a GPR survey and highlight the need for further studies investigating other settings.

Remote Sensing, 2022

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Remote Sensing, 2021

The Benedictine Convent of Saint John at Müstair is a UNESCO World Heritage Site located in the e... more The Benedictine Convent of Saint John at Müstair is a UNESCO World Heritage Site located in the eastern part of Switzerland close to South Tyrol’s border (Italy). Known as a well-preserved Carolingian building complex housing Carolingian and Romanesque frescoes, the convent has received much academic attention. However, all research activities so far have been concentrated on the area enclosed by the convent’s walls, even though the neighbouring fields to the east and south are also part of the convent’s property. This paper reports on the archaeological magnetic and ground-penetrating radar surveys of these areas, executed as part of a pilot project exploring the convent’s immediate environment. At present, these fields are used for agriculture and located on a massive alluvial fan of the mountain stream Valgarola. Dense geophysical sampling revealed an intricate network of distributary channels with stream and mudflow deposits, constituting a natural border of the convent’s territory. In addition to different field systems, a newly discovered broad pathway appears to be an original Roman road. Numerous structural elements, mapped within the convent’s walls, could be attributed to specific building phases. Over 40 large and deep burial shafts, arranged in three rows, were discovered outside the convent’s burial ground. Their specific design and arrangement are characteristic of early medieval burials, such as those of the 6th century Lombards on the edge of the eastern Alps.

Österreichische Zeitschrift für Kunst und Denkmalpflege, Jun 24, 2021

The surface of most heritage objects holds important clues about their creation. To answer specif... more The surface of most heritage objects holds important clues about their creation. To answer specific research questions about the creation of a mural painting located in the Bishop's Gate porch of St. Stephen's Cathedral in Vienna, the three-dimensional geometry of the entire painted surface was digitised in minuscule detail using thousands of overlapping photographs. After a brief introduction to the basics of image-based surface digitisation, this paper describes in detail how the photographs were taken and how they were processed to create a digital three-dimensional surface. To appreciate the subtle surface variations encoded in this digital model, various relief visualisation techniques have been applied. The paper ends with a discussion of the new insights into the creation of this painting that are enabled by these visualisations.

Remote Sensing, 2021

Large parts of the urban layout of the abandoned Roman town of Bassianae (in presentday Serbia) a... more Large parts of the urban layout of the abandoned Roman town of Bassianae (in presentday Serbia) are still discernible on the surface today due to the deliberate and targeted quarrying of the Roman foundations. In 2014, all of the town's intramural (and some extramural) areas were surveyed using aerial photography, ground-penetrating radar, and magnetometry to analyze the site's topography and to map remaining buried structures. The surveys showed a strong agreement between the digital surface model derived from the aerial photographs and the geophysical prospection data. However, many structures could only be detected by one method, underlining the benefits of a complementary archaeological prospection approach using multiple methods. This article presents the results of the extensive surveys and their comprehensive integrative interpretation, discussing Bassianae's ground plan and urban infrastructure. Starting with an overview of this Roman town's research history, we present the details of the triple prospection approach, followed by the processing, integrative analysis, and interpretation of the acquired data sets. Finally, this newly gained information is contrasted with a plan of Roman Bassianae compiled in 1935.

The processing of aerial imagery acquired over Montarice hill (central Adriatic Italy) during the... more The processing of aerial imagery acquired over Montarice hill (central Adriatic Italy) during the Potenza Valley Survey will form the key focus of this paper. Since this site has repeatedly revealed itself in terms of interesting vegetation and soil marks, the most interesting footage, acquired during two observer-directed sorties more than a decade ago, will be examined. First, the potential of state-of-the-art image-based modelling (IBM) techniques is explored to create high-resolution orthophotographs from these analogue frame images. Since dense image matching-as a part of IBM-allows to model the vegetation surface at the moment of the photographic survey, a geometrical three-dimensional representation of the plant canopy is possible. This contribution shows how the latter can be the focus of its own information extraction process, using techniques mainly developed in the field of airborne laser scanning. However, the true interpretative power lies in the combination of the co-registered spectral and geometrical dimensions of the vegetation. Using Montarice as a case study, it will become clear that crop height data allows for powerful visualisations that can aid and even alter interpretative mapping that is solely based on colour differences in orthophotographs.

$Research paper thumbnail of Resolving some spatial resolution issues - Part 2: When diffraction takes over$

In part 1 “Between line pairs and sampling distances” of this split entry on spatial resolution, ... more In part 1 “Between line pairs and sampling distances” of this split entry on spatial resolution, it was mentioned that this second part would focus on some fundamental laws of electromagnetic radiation to shed more light on the concepts of spatial resolving power and spatial resolution. The numbering of chapters, illustrations and equations simply continues so that one can easily merge both parts into one larger text. However, and in contrast to what was initially stated, this entry will not be the final part about spatial resolution. Because there is quite some, but still basic, ground to cover once the physical views on spatial resolution start to complement the geometrical ones, there will also be a third part. This approach also prevents any of these entries from becoming too lengthy.

Leonardo da Vinci must have been aware that Columbus discovered new territories in the West. Unti... more Leonardo da Vinci must have been aware that Columbus discovered new territories in the West. Until now, no material evidence had been found to substantiate this assumption. Here we show that Leonardo not only read Amerigo Vespucci's letter (derived from a painted star constellation), but that he even drew a map including the New World, a drawing which was previously interpreted as a depiction of the Moon. Finally, Leonardo engraved his notion of this new continent on an ostrich egg globe (now known as the Da Vinci Globe) and made a copper cast of this. Both the cosmographic and carto-graphic clues demonstrate that Leonardo da Vinci knew about the fourth continent, to be named "America" in 1507, less than a decade after Columbus embarked upon its shores. This expansion of Leonardo's cartographic legacy comes at a time of increased interest for such multidisciplinary insights, as the world commemorates in 2019 the 500 th anniversary of his death.

Lebenswelten zwischen Archäologie und Geschichte - Festschrift für Falko Daim zu seinem 65. Geburtstag, 2018

Carnuntum Jahrbuch, 2018

Over the course of four years (2012–2015) the Ludwig Boltzmann Institute for Archaeological Prosp... more Over the course of four years (2012–2015) the Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (LBI ArchPro), in collaboration with the Central Institute for Meteorology and Geodynamics (ZAMG) and on behalf of the provincial government of Lower Austria, has conducted the comprehensive, non-invasive archaeological prospection project entitled “ArchPro Carnuntum”. The purpose of this project has been to generate a basis for future archaeological research and the sustainable spatial planning and cultural heritage management in the area. By combining new information on buried archaeology using different aerial and ground-based archaeological prospection methods and a thorough archaeological interpretation of the combined data within the framework of a GIS environment, it was possible to document Carnuntum’s archaeological heritage, which is threatened by a dramatic increase in erosion and destruction through ploughing, infrastructure development and looting by treasure hunters. In total, an area covering eight square kilometres was explored at very high sampling resolution using magnetic prospection methods, while 2.5 square kilometres were mapped with ultra-high resolution ground-penetrating radar measurements. The results of the project and the spatio-temporal analysis and interpretation of the prospection data are presented here as a preliminary report.

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Feb 1, 2019

In the second half of the 19th and early 20th century, sheep shepherds have built dry-stone shelt... more In the second half of the 19th and early 20th century, sheep shepherds have built dry-stone shelters all over the Slovene Kras (or Karst) region. Despite being made out of stones that are interlocked without the use of any binding material, many of these vernacular constructions survived – even though sometimes only partially – the ravages of time. The fact that over one hundred fifty shepherd shelters are currently known is mainly due to the craftsmanship of their builders and thanks to (and even despite) their present location. A majority of these stone constructions can be found in areas that are nowadays forested, thus shielding them from weather-related or anthropogenic damage (because they are difficult to spot). This paper reports on the geometric documentation of those shelters using a photogrammetric computer vision pipeline, thereby mainly focussing on the difficulties that were encountered during this process. However, such image-based modelling approaches merely yield digital three-dimensional (3D) approximations of the shelters’ surface geometry (along with some sub-optimal colour data). Although these 3D surface models might be suitable to digitally preserve vulnerable vernacular buildings to some extent, they do not magically advance our understanding of them. The second part of this article focuses, therefore, on the extraction of archaeological information from these digital 3D constructions. More specifically, the total amount of stones, the total building time and the building cost regarding caloric energy expenditure are estimated for each of the digitised shelters. Although this assessment of architectural energetics provided useful insight into the building efforts and nutrient uptake of the shepherds, it also revealed many assumptions and shortcomings that often characterise archaeological information extraction from digital 3D models of buildings.

One of the most common words in the remote sensing (or even general imaging) literature is ‘resol... more One of the most common words in the remote sensing (or even general imaging) literature is ‘resolution’. Despite its abundant use and because the concept is often misjudged as uncomplicated, most modern literature relies on rather sloppy ‘resolution’ definitions that sometimes even contradict each other within the same text. In part, this confusion and misconception arises from the fact that technical as well as broader, application‐specific explanations for resolution exist, both of them even relying on different ways to describe resolution characteristics. As a result, the term ‘resolution’ has been used for many years as a handy go‐to term to cover many concepts: “this satellite produces images with a resolution of 30 m”; “there is an increasing number of high‐resolution camera sensors on the market” or “the resolution of the human eye is coarser than an eagle’s eye”. Nowadays, one might wonder if resolution is a particular image characteristic, a property of the imaged scene or instead related to the imaging sensor or maybe the camera’s lens.
It is thus fair to say that the technical concept of resolution – or more specifically spatial resolution – and all its implications are commonly poorly understood, which leads to many popular, accepted but completely wrong statements. In the photographic literature, a widespread example is to refer to the total number of camera image pixels (i.e. the pixel count) as the image resolution of that specific digital camera. This is erroneous since the same 24‐megapixel camera can capture a photograph of an Attic black‐figure amphora as well as a complete submerged Greek temple. The resulting two photographs, although both are counting 24 megapixels, might reveal scene details of 0.01 cm and 2 cm respectively. In the remote sensing community, a prevalent misconception is that a satellite image with a 1 m resolution automatically means that we can recognise all objects in that image which have a width equal to or larger than 1 m.
In this two‐part entry of our series, we will combine simple geometrical relationships (part 1) and fundamental laws of electromagnetic radiation (part 2) to shed some light on the term spatial resolution and explain its difference with the related concept of spatial resolving power. Similar to the previous two entries, this two‐pieced text can only scratch the surface of this very complex topic. Notwithstanding, the aim is still to provide solid definitions and enough background knowledge to easily correct many of the “common knowledge” but ill‐founded statements such as the ones mentioned above.

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2018

Acquiring photographs as input for an image-based modelling pipeline is less trivial than often a... more Acquiring photographs as input for an image-based modelling pipeline is less trivial than often assumed. Photographs should be correctly exposed, cover the subject sufficiently from all possible angles, have the required spatial resolution, be devoid of any motion blur, exhibit accurate focus and feature an adequate depth of field. The last four characteristics all determine the " sharpness " of an image and the photogrammetric, computer vision and hybrid photogrammetric computer vision communities all assume that the object to be modelled is depicted " acceptably " sharp throughout the whole image collection. Although none of these three fields has ever properly quantified " acceptably sharp " , it is more or less standard practice to mask those image portions that appear to be unsharp due to the limited depth of field around the plane of focus (whether this means blurry object parts or completely out-of-focus backgrounds). This paper will assess how well-or ill-suited defocus estimating algorithms are for automatically masking a series of photographs, since this could speed up modelling pipelines with many hundreds or thousands of photographs. To that end, the paper uses five different real-world datasets and compares the output of three state-of-the-art edge-based defocus estimators. Afterwards, critical comments and plans for the future finalise this paper.

Heritage, 2022

Remote Sensing, 2022

Remote Sensing, 2021

Österreichische Zeitschrift für Kunst und Denkmalpflege, Jun 24, 2021

Remote Sensing, 2021

$Research paper thumbnail of Resolving some spatial resolution issues - Part 2: When diffraction takes over$

Lebenswelten zwischen Archäologie und Geschichte - Festschrift für Falko Daim zu seinem 65. Geburtstag, 2018

Carnuntum Jahrbuch, 2018

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Feb 1, 2019

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2018

This is the book of abstracts created for the goINDIGO 2023 international graffiti symposium orga... more This is the book of abstracts created for the goINDIGO 2023 international graffiti symposium organised in the framework of the academic project INDIGO.

This is the book of abstracts created for the goINDIGO 2022 international graffiti symposium orga... more This is the book of abstracts created for the goINDIGO 2022 international graffiti symposium organised in the framework of the academic project INDIGO.

MDPI - ISBN: 978-3-0365-1376-8, 2021

Despite the many (r)evolutions in remote sensing technology over the past three decades, integrat... more Despite the many (r)evolutions in remote sensing technology over the past three decades, integration in archaeological practice and theory has sometimes been limited by reliance on practice and theory imported from other disciplines, without questioning or deep understanding. This collection of papers aims to contribute to the exploration of developing practice and theory in remote sensing archaeology for the 21st century. The scope of this volume is the use of remotely sensed data from either air- or spaceborne platforms for the benefit of archaeology and cultural heritage in general, with a specific focus on better defining the roles and contexts that detail why archaeologists may apply remote sensing techniques. With this focus, it is our hope that remotely sensed data will be better and more intrinsically integrated into the symbiosis of archaeological practice and theory.
The editorial for this volume suggests that many aspects of archaeological practice can be characterised as ‘beg, borrow and steal’. This collection provides the reader with thoughtful papers that contribute to the development of archaeological remote sensing as a mature interdisciplinary field characterised by explicit and theoretically engaged approaches to understanding the past.

The process which gave rise to this book began in the spring of 2006, in the wake of the first ‘B... more The process which gave rise to this book began in the spring of 2006, in the
wake of the first ‘Broadening Horizons’ conference on multidisciplinary
landscape study (Ghent, Belgium). At the time, we - a group of ‘mere’ doctoral
students - were already happy enough to have pulled off the organisation of an
international conference; the idea of having it flow over into a publication was
probably the farthest from our mind as we were enjoying our well-earned
closing dinner. But after the dust of those hectic final weeks had settled and the
ideas put forward in the presentations had sunk in, we began to play with the
idea nonetheless.
And so a year later, we are pleased to present this volume on behalf of all
who contributed before, during and after the conference. We expressly opted to
make the leap from a ‘conference proceedings’ to a stand-alone volume of
selected essays, as we felt the latter would better lend itself to the distribution of
the interdisciplinary and supra-regional scope that we deem an absolute
necessity of landscape research. The selected texts as a result take diverse angles
at tackling the ‘landscape problem’, covering ground from the Central
Mediterranean to the Middle-East and from semi-automated remote sensing to
cuneiform historical geography. By including contributions from beginning and
more established researchers alike, we aimed to produce a volume of potential
interest to both the student and the specialist. Aside from offering concrete
examples of applied methodology, we hoped the book might through this
intentionally broad gaze also help further interdisciplinary awareness.
Thanks go out firstly to the authors who contributed to this volume; we also
regret that, given the preset publication size, not all papers which we received
could be included. A special acknowledgement must be made to Prof. Tony
Wilkinson, whose contribution to the conference and key-note lecture proved
more stimulating than we could ever have hoped for and who has kindly agreed
to write the introduction to this book. Finally, we thank our colleagues here at
the university: Sarah Deprez, Joke Dewulf, Tanja Goethals and Wouter Gheyle;
for their shared enthusiasm, hours of overtime and combined expertise which
made Broadening Horizons possible.

Méthodes d’enregistrement des données en archéologie, Dec 18, 2019

The Encyclopedia of Archaeological Sciences, Dec 5, 2018

Satellite‐based imagers have been delivering data products that offer new possibilities to study ... more Satellite‐based imagers have been delivering data products that offer new possibilities to study (archaeological) landscapes via remote recording of the Earth's spectral reflectance (and to some extent thermal emittance) since the 1970s. Although much declassified panchromatic spy footage and some more recent commercial multispectral imagery has already been used with great success in archaeological research, it is expected that the importance and utility of multi‐ and hyperspectral (together denoted spectral) spaceborne imagery will become more important in the near future. This is mainly due to the constant and simultaneous improvements in the spatial and spectral resolving power of these imaging systems. Together with the temporal and radiometric properties, these four characteristics define any remotely‐sensing spaceborne spectral imaging platform. Thus, these parameters allow one to decide on the archaeological usability of the footage acquired.

The Encyclopedia of Archaeological Sciences, Nov 26, 2018

Since the invention of photography, a variety of approaches have made noninvasive imaging for cul... more Since the invention of photography, a variety of approaches have made noninvasive imaging for cultural heritage applications possible. Multi‐ and hyperspectral imaging (together denoted spectral imaging) are two techniques that evolved from conventional color photography, having overcome its spectral limitations. Instead of imaging in three broad spectral bands, multispectral imaging acquires data in up to ten more or less equally wide and nonoverlapping spectral bands. Hyperspectral imaging goes beyond the multispectral approach by generating images in tens to hundreds of narrow, contiguous (i.e., adjacent but not overlapping) spectral bands. Most spectral imaging techniques are limited to the optical electromagnetic spectrum and acquire the reflected portion of the radiation that is used to illuminate the scene. However, imaging the emitted thermal radiation or active techniques based on laser scanners do exist as well. In cultural heritage, all these techniques are used to gain a better, noninvasive insight into the chemical and physical properties of the object(s) under investigation.

Legionslager und Canabae Legionis in Pannonien, 2016

I siti archeologici della Vallata del …, Jan 1, 2006

Graffiti sind eine kurzlebige und polarisierende Form unseres kulturellen Erbes. Graffiti verärge... more Graffiti sind eine kurzlebige und polarisierende Form unseres kulturellen Erbes. Graffiti verärgern, erfreuen, provozieren und regen zum Nachdenken an. Der Wiener Donaukanal zählt zur längsten ununterbrochenen Graffiti-Landschaften weltweit und ist damit einer der wichtigsten Graffiti-Hotspot. Heutzutage bedeutet das Sprayen eines neuen Graffito am Donaukanal meist die teilweise oder vollständige Zerstörung eines darunterliegenden Graffito. Daher verändert sich auch die Graffiti Landschaft entlang des Donaukanals täglich. Das Projekt INDIGO hat es sich zum Ziel gesetzt, die 13 km Graffiti-bedeckten Oberflächen des Donaukanals systematisch zu dokumentieren und in Form einer interaktiven und aktuellen online 3D-Plattform der Wissenschaft und einer breiteren Öffentlichkeit zugänglich und auch für kommende Generationen erlebbar zu machen. Am Ende des Projekts INDIGO soll es unter anderem möglich sein, einen virtuellen Spaziergang entlang des Donaukanals zu machen und längst übersprühte Graffiti im Detail zu analysieren.

Die 3D Geometrie des Donaukanals wird durch Structure from Motion (SfM) und mittels eines Multi-View Stereo (MVS) Ansatzes aus ca. 27000 Fotos abgeleitet, die im Herbst 2021 aufgenommen wurden. Zusätzlich zu den Fotos wurden mehr als 500 Passpunkte entlang des Donaukanals terrestrisch vermessen, um die Georeferenzierung des 3D Modells zu gewährleisten und akkumulierte Drifteffekte der berechneten Kameraorientierungen zu minimieren. Eine der größten Herausforderungen von INDIGO ist es, alle neuen Graffito-Fotos in ein (ortho-)rektifiziertes, georeferenziertes und farbtreues Bild zu verwandeln. Um dies zu ermöglichen, entwickelt INDIGO maßgeschneiderte und hochautomatisierte Workflows, die der Öffentlichkeit als open-source Software für andere Dokumentationsprojekte kostenlos zur Verfügung gestellt werden. Die Orthorektifizierung erfolgt mittels eines sogenannten inkrementellen SfM Ansatzes, in dem die neuen Bilder nach und nach in den bestehenden Bündelblock hinzugefügt und orientiert werden. Um dieses Verfahren zu beschleunigen nutzt INDIGO Messdaten von GNSS und IMU Sensoren, die auf der Kamera montiert sind.

In diesem Vortrag werden die Ziele des INDIGO Projekts vorgestellt und diverse photogrammetrischen Lösungsansätze präsentiert, die das digitale Fortbestehen der Graffiti Landschaft am Donaukanal sicherstellen.

INDIGO is a two-year, ÖAW-funded research project that aims to document, disseminate and analyse ... more INDIGO is a two-year, ÖAW-funded research project that aims to document, disseminate and analyse the graffiti-scape along Vienna's Donaukanal. INDIGO's prime data are digital photos because they capture the geometrical, spectral, and certain temporal aspects of newly created graffiti. To speed up their archiving and photogrammetric processing, INDIGO developed a compact piece of hardware that sits on top of the camera to record centimetre-accurate camera coordinates for each photo. This poster details the hard- and software of our cost-effective geocoding solution.

Graffiti are a short-lived and polarising form of cultural heritage. The graffiti-focused researc... more Graffiti are a short-lived and polarising form of cultural heritage. The graffiti-focused research project INDIGO, funded by ÖAW, documents and digitally preserves (almost) every graffito created along Vienna’s Donaukanal, one of the world’s largest graffiti hotspots. A central aim of INDIGO is the creation of a georeferenced orthophoto for each graffito, which digitally geo-localizes the graffito in 3D space and facilitates the interpretation in a spatial context. The poster explains the “orthophoto” concept and details how INDIGO uses modern photogrammetric techniques to derive them automatically.

Since the astronomer and composer Sir Frederick William Herschel (1738-1822) discovered in 1800 t... more Since the astronomer and composer Sir Frederick William Herschel (1738-1822) discovered in 1800 the infrared portion of the electromagnetic spectrum, many other scientists became interested in this kind of invisible radiation. It lasted, however, until 1904 for the first near-infrared photograph to be taken. From the 1930s onwards, this unusual type of imaging was practised more elaborately, specifically to examine damaged and censored writings or study blood patterns for medical purposes.
After 1935 – the year in which one of the earliest infrared aerial photographs was taken from a stratosphere balloon – the trend was set. Less than a decade afterwards, aerial infrared colour film became extensively used for its camouflage detection capabilities in WWII. Today, orbital and aerial NIR recording serves a great number of applications, being intensively used by the military as well as the scientific fields of hydrology, geology, forestry and archaeology.
Up till now, NIR radiation was mostly captured in an analogue way by infrared sensitive plates or film emulsions (black-and-white or colour), or digitally by satellites or high-tech multispectral sensors. For various reasons (cost, resolving power, lack of hardware etc.), aerial archaeologists use(d) the analogue NIR approach to study their objects (some examples are – amongst others – the work of Bradford, Strandberg, Solecki, Edienne and Martin).
Such a film-based workflow is however very error-prone, as the emulsions need to be stored cooled and developed by specialised labs directly after exposing them. Moreover, determining the right exposure is not as straightforward as with conventional/standard (i.e. visible light) photography. Together with some ignorance and/or lack of knowledge about the subject, this critical imaging process severely restricted NIR radiation to be captured by aerial archaeologists so far.
However, this changed completely with the advent of digital cameras. As their sensors are very sensitive to NIR radiation, the whole process of taking NIR photographs is much less of a cumber stone. The poster under consideration wants to show how NIR imagery can be taken with normal (and converted) digital cameras, what the images look like, compare the advantages (and disadvantages) to normal aerial imaging (e.g. haze penetration, enhanced clarity of detail and visualization of stressed vegetation) as well as outline a basic approach of NIR image processing.

Due to the constant development of new and better imaging sensors which operate according to diff... more Due to the constant development of new and better imaging sensors which operate according to different physical principles, the need arises for a meaningful combination of all these imaging data. For many years, various research fields have been trying to integrate imagery of different modalities – not at least in the medical and geoscience communities – to facilitate a better understanding and interpretation of particular phenomena. This topic is also becoming increasingly applicable in archaeology, as imagery and data sources become more diverse and complex.

In order to implement such an approach, a dedicated MATLAB toolbox TAIFU (the Toolbox for Archaeological Image Fusion) has been created. TAIFU serves as a platform for testing of current state-of-the-art image fusion methods and facilitates the development of new data integration routines. This toolbox is thus designed to benefit archaeological interpretive mapping of diverse prospection datasets. For example, the data from a magnetogram can be fused with an aerial image to aid the archaeologist in correlating feature locations for a more trustworthy information extraction and better interpretation of hidden geo-cultural features.

TAIFU is currently programmed in such a way that the user has to load two images. Upon import, the toolbox verifies and stores the metadata of the images (such as georeferencing, Exif and IPTC tags). Subsequently, a variety of image fusion processes (some with additional options) can be chosen to create a fused image from the initial displayed images. As a pre- or post-fusion step, the user can also choose among a variety of contrast enhancement algorithms. When a useful result is obtained, the fused image can be saved with all the proper metadata embedded, although the latter can also be stored as a sidecar ASCII file. These new metadata do not only originate from both input images, they also contain data about the contrast enhancement and fusion algorithms that were used to obtain the final result.

At this stage, TAIFU is still in development. So far, about thirty image fusion methods have been implemented next to two different image fusion metrics. Although a visual assessment of the result will normally determine which method is best suited for a specific case, these metrics can help to identify which fusion approaches preserve most information of both input images.

This talk will first introduce various forms of graffiti and define shortcomings in much of the scholarly research on (modern and ancient) graffiti. At that point, the stage is set to introduce project INDIGO. INDIGO (IN-ventory and DI-sseminate G-raffiti along the d-O-naukanal) was a two-year academic project launched in September 2021 through funding from the Austrian Academy of Sciences. INDIGO pushed the status quo in documenting, managing and understanding extensive graffiti-scapes, for which the distinctive graffiti along circa 13 km of Vienna's Danube Canal served as a case study.

INDIGO wanted to ensure this graffiti-scape's digital survival and interdisciplinary investigation by creating a long-term, accurate, exhaustive, open-access and interactive online archive. Therefore, this talk will mainly detail the methodological and logistical developments created to deal with INDIGO's technical- and more humanistic-oriented aspects. However, some project bottlenecks and non-achievements will also be discussed.

Colourful and quickly changing: contemporary graffiti can be considered the chameleon skin of any... more Colourful and quickly changing: contemporary graffiti can be considered the chameleon skin of any urban landscape. Many modern graffiti might evoke the feeling of violating basic principles of acceptable social behaviour while providing colour to a city and displaying artistic skill. This tension between vandalism and art explains why contemporary graffiti can be so polarising and why they intrigue.
This talk will first introduce various forms of graffiti and make a case for their status as cultural heritage. After defining shortcomings in much of the scholarly research on (modern and ancient) graffiti, the stage is set to introduce project INDIGO. INDIGO (IN-ventory and DI-sseminate G-raffiti along the d-O-naukanal) was a two-year academic project launched in September 2021 through funding from the Austrian Academy of Sciences. INDIGO pushed the status quo in inventorying and understanding extensive graffiti-scapes, for which the distinctive graffiti along circa 13 km of Vienna's Danube Canal served as an example.
INDIGO wanted to ensure this graffiti-scape's digital survival and interdisciplinary investigation by creating a long-term, accurate, exhaustive, open-access and interactive online archive. The majority of this talk will, therefore, detail the methodological and logistical developments created to deal with this project's technical- and more humanistic-oriented aspects.

Continuous and discrete digital representations of a surface can either be constructed from scrat... more Continuous and discrete digital representations of a surface can either be constructed from scratch in a Geographic Information System, Computer-Aided Design or Computer Graphics environment, or obtained through digitising the surface geometry of an existing physical object or scene. Amongst the many application-driven techniques developed by multiple disciplines, one possible approach to surface digitisation is via photographs (or more general images), commonly referred to as Image-Based (3D) (surface) Modelling or simply IBM.
IBM encompasses different techniques, but most rely on object or scene photographs taken from different locations to extract digital surface data. IBM has been the focus of both the photogrammetric and computer vision fields. However, hybrid photogrammetric computer vision-based approaches like Structure from Motion (SfM), Simultaneous Location/Localisation And Mapping (SLAM), and Multi-View Stereo (MVS) have become commonplace in cultural heritage documentation over the past 1.5 decades. This rapid technology uptake also led to an ever-increasing and continuous stream of new literature, filled almost exclusively with software comparisons or developments towards faster and more automated approaches.
However, one thing stands out in this story: the surprisingly little attention devoted to the input of all these algorithms: the photographs. Are digital cameras so ubiquitous, and have algorithms become so robust that academics stopped paying attention (if they ever really did?) to how a camera’s operating principles tailor the pixels they generate?
This talk will go back to the basics of all IBM approaches – pixels – and detail how camera architectures and settings affect their positional and spectral values. Various case studies will illustrate how the resulting sub-optimal or parasitic pixels can negatively affect the IBM output and which measures can mitigate such issues. Ultimately, the paper hopes to incentivise more awareness for, and academic research on, the basics of cultural heritage IBM: photography.

This presentation focuses on Project INDIGO's objectives and structural framework. It explores th... more This presentation focuses on Project INDIGO's objectives and structural framework. It explores the development of the definitions of graffito within the project, and points out the significance of integrating a distinction between the physical graffito and its digital representative. We also leverage insights and data from other graffiti research projects, illustrating our collaborative efforts to enhance graffiti metadata standards.

Colourful and ever-changing: Graffiti can be considered the urban chameleon skin. At the Donaukan... more Colourful and ever-changing: Graffiti can be considered the urban chameleon skin. At the Donaukanal (Eng. Danube Channel), Vienna's central waterway and one of the largest and most active graffiti-scapes worldwide, this metaphor applies like hardly anywhere else. Every day a multitude of graffiti is destroyed by the creation of new works. Recently, efforts have been made to mitigate this constant loss of cultural heritage along the Donaukanal by systematically documenting the graffiti, mainly using photography and photogrammetry. However, keeping track of the newly added works is very time-consuming and often like finding needles in a haystack, considering the large extent and high volatility of the monitored area. Thus, an automated graffiti change detection would significantly reduce the effort and avoid overlooking graffiti.
This contribution outlines the main challenges in image-based change detection for cultural heritage and proposes a hybrid graffiti change detection method. The investigated method exploits and combines an established pixel-based change detection algorithm, the Iteratively Multivariate Alteration Detection, with a novel descriptor-based method. The latter relies on image features, rather than pixels as analysis unit and can robustly filter false alarms from the high-performing but noise-prone pixel-based approach. Overall, the results indicate that the proposed method can largely automate image-based change detection of graffiti-scapes. It can uncover graffiti-related changes and robustly distinguish them from other image differences such as shadows but tends to overlook small-scale graffiti, indicating the need for further fine-tuning.

Colourful and ever-changing: Graffiti can be considered the urban chameleon skin. At the Donaukan... more Colourful and ever-changing: Graffiti can be considered the urban chameleon skin. At the Donaukanal (Eng. Danube Channel), Vienna's central waterway and one of the largest and most active graffiti-scapes worldwide, this metaphor applies like hardly anywhere else. Every day a multitude of graffiti is destroyed by the creation of new works. Recently, efforts have been made to mitigate this constant loss of cultural heritage along the Donaukanal by systematically documenting the graffiti, mainly using photography and photogrammetry. However, keeping track of the newly added works is very time-consuming and often like finding needles in a haystack, considering the large extent and high volatility of the monitored area. Thus, an automated graffiti change detection would significantly reduce the effort and avoid overlooking graffiti.
In this presentation, the main challenges in image-based change detection for an extensive graffiti-scape are outlined. Furthermore, we will showcase a camera-based monitoring framework that provides a robust foundation of data, which serves as input for a novel hybrid method of image-based change detection. The investigated method exploits and combines an established pixel-based change detection algorithm, the Iteratively Multivariate Alteration Detection, with a descriptor-based method. The latter relies on image features rather than pixels as an analysis unit and can robustly filter false alarms from the high-performing but noise-prone pixel-based approach. Overall, the results indicate that the proposed method can largely automate image-based change detection of graffiti-scapes. It can uncover graffiti-related changes and robustly distinguish them from other image differences such as shadows but tends to overlook small-scale graffiti, indicating the need for further finetuning.

This presentation will introduce the online platform UrbanChameleon, which we have been developin... more This presentation will introduce the online platform UrbanChameleon, which we have been developing to facilitate the analysis and dissemination of contemporary graffiti in Vienna, Austria. The platform is designed to visualise and analyse data collected in the scope of project INDIGO, which is funded by the Heritage Science Austria programme of the Austrian Academy of Sciences (ÖAW) and focuses on circa 13 km of graffiti-covered surfaces along the Danube Canal.
Project INDIGO utilises OpenAtlas, an open-source, web-based database system that leverages the CIDOC CRM (Conceptual Reference Model) to manage and visualise historical, archaeological, and cultural heritage data. The CIDOC CRM is an international standard ontology for exchanging and integrating heterogeneous cultural heritage data and information, enabling the representation of complex relationships and events in a structured manner. The online platform seamlessly connects to OpenAtlas through an API (Application Programming Interface), a set of protocols and tools that enables different software applications to communicate and share data. This integration allows the platform to efficiently fetch and incorporate graffiti data, ensuring a comprehensive and user-friendly exploration experience.
A key feature of the online platform is a 4D viewer that allows users to visualise graffiti in its original location and context, as well as track its temporal changes. This is achieved via Cesium-based tools, open-source software libraries designed to create and visualise 3D geospatial data in web applications. Known for their powerful and flexible capabilities, these tools enable the rendering of high-quality, realistic 3D environments. By incorporating Cesium-based tools into UrbanChameleon, the platform provides immersive and detailed visualisations of graffiti, making it easier for users to understand their spatial context.
UrbanChameleon benefits from enhanced stability, performance, and scalability, providing a seamless and efficient user experience when exploring graffiti in Vienna. The platform features a user-friendly interface with an interactive map component, offering valuable insights into the distribution and context of contemporary graffiti. By focusing on the core functionalities and user experience, the platform allows researchers to study and understand the cultural heritage and significance of graffiti in an intuitive manner.
In conclusion, the UrbanChameleon platform provides researchers with a comprehensive tool to explore, search, and filter graffiti in Vienna, specifically at the Hall of Fames along the Danube Canal. It enables them to uncover layers of meaning and contextual significance. To truly engage with those graffiti records and unlock their analytical potential, smooth data streaming and proper treatment of the temporal dimension are key, which UrbanChameleon enables via various technical means. In the end, we hope that the platform can be used to digitally explore the colourful walls of the Danube Canal over time and space, but also facilitate the creation of valuable insights into the evolution of graffiti and its role in our contemporary society.

A digital photo file consists of two (or more) parts: one contains the pixel values that encode t... more A digital photo file consists of two (or more) parts: one contains the pixel values that encode the visible light reflected from (or emitted by) real-world objects, while one or more file segments hold the metadata. Storing these photo metadata is enabled by various standards. For instance, the Exif (Exchangeable image file format) standard records technical photo metadata such as the camera's serial number and model, lens aperture and focal length, shutter speed, possible flash compensation, and the date plus time of photo creation. All Exif-defined tags are created by the camera and stored simultaneously upon photo creation.
Besides Exif, IPTC Photo Metadata exist. These metadata – structured in a standard proposed by the IPTC – are the widely accepted norm for storing administrative, copyright, and descriptive information in images (unedited photos, edited pictures, and AI-generated images). In its 2019.1 version, the IPTC Photo Metadata Standard enabled the storage of image regions. These image regions are shapes like rectangles, circles or any possible polygon that can be marked and saved within an image.
Since the authors are unaware of a tool that allows straightforwardly working with such image regions, and given their potential for graffiti photo annotation, project INDIGO created GRAPHIS. GRAPHIS is an open-source, freely available Python-based software to create image regions, annotate them with graffiti descriptions or transcriptions, and visualise them. The backbones of GRAPHIS are an SQLite database and ExifTool. Besides storing links to the photos of interest, the SQLite database keeps track of every image region operation. This principle enables users to start/exit the software at will without the risk of losing work. It also enables collaboration on various photo collections, as each can have its database. At any moment, the user can write the image regions back into the photo's metadata segment, an operation for which GRAPHIS utilises ExifTool.
GRAPHIS adheres to the IPTC standard at every stage. For example, the IPTC stipulates that every region ideally features a concept that clarifies the role and content type of the region. These concepts must come from a controlled vocabulary: a confined list of terms, each with a definition and a URI. Since none of IPTC's controlled vocabularies features graffiti-related concepts, project INDIGO also created a separate GRAPHIS Image Region vocabulary to contain all relevant concepts. This presentation will first explain the inner workings of GRAPHIS and its underlying vocabulary, after which the software and its functionality will be shown live on some graffiti photographs.

Project INDIGO aims to document, disseminate and analyse the contemporary graffiti-scape surround... more Project INDIGO aims to document, disseminate and analyse the contemporary graffiti-scape surrounding the Viennese Danube Canal (Austria). Despite its focus on present-day graffiti, INDIGO is considered an archaeological project. The authors take the stance that archaeology is an academic discipline trying to understand (our complex relationships with) the material remains of the stratified past, whether that past was centuries (i.e. the remote or ancient past) or days (i.e. the contemporary or recent past) ago. From this point of view, it should not be surprising that INDIGO applies (and improves) various tools and workflows commonly used for acquiring and managing data in more conventional archaeological projects.
However, outsiders might find it strange to hear that a discipline examining space- and time-bound anthropogenic activities has only developed tools to properly analyse and visualise the two-dimensional spatial component of archaeological data. Effective practical approaches to handling the third spatial dimension, let alone the temporal aspect, are generally lacking. INDIGO wants to leverage its focus on "less old" things to advance the management, visualisation and analysis of the uncertain spatio-temporal boundaries characterising "older things".
Post-depositional processes like erosion, ploughing, animal digging, and soil formation increasingly bias and fuzzyfy the spatial and temporal information obtainable from common archaeological remains like Bronze Age ditches or Celtic ceramics. Because modern artefacts like graffiti are typically less influenced by post-depositional processes, and since their continuous "layering" (which archaeologists call stratification) can be documented in situ, contemporary graffiti-scapes lend themselves well to develop archaeologically-relevant ways of dealing with spatio-temporal complexities.
Despite considerable effort, monitoring the dynamic nature of graffiti along the Danube water channel still poses problems concerning data quality and completeness. INDIGO's initial spatial and temporal (meta)data only accounts for when (and if) a graffito was photographed. Whether the graffito production occurred hours or days before typically remains unknown, while some graffiti even go undocumented. INDIGO relies on a polygon for each graffito to address these challenges. This polygon is first indicated on – and stored within – each graffito's overview photo via the bespoke software GRAPHIS. AUTOGRAF – another tool coded within INDIGO – transforms this image-bound two-dimensional polygon into a three-dimensional geometric entity correctly located in a real-world coordinate reference system.
A GeoJSON file stores each graffito-polygon. This human- and machine-readable file complements the polygon coordinates with various attribute fields for temporal data. Some fields can be populated upon creating the polygon, while other temporal data might be automatically inferred from subsequent polygons. At the end of the process, each polygon should provide a digital, nuanced representation of one graffito's spatial and temporal dimensions. Through their combination, we hope that INDIGO's online visualisation platform can extract the various explicit or implicit spatial and temporal relationships among the thousands of documented graffiti.

Colour is a powerful communication element of graffiti and many other forms of cultural heritage.... more Colour is a powerful communication element of graffiti and many other forms of cultural heritage. This importance of colour notwithstanding, the documentation of cultural heritage typically only focuses on the geometrical aspects and seldom on the spectral characteristics of the object. This is party because colour and the science of colour – called colourimetry – are non-trivial. In addition, it remains tough to capture accurate colour data with standard digital cameras, certainly when the illumination conditions are constantly varying, like in outdoor photography. As a result, not many research teams have furthered accurate colour data acquisition with regular photo cameras for spectrally documenting cultural heritage. Given the importance of colour in graffiti, obtaining correct image colours is essential from an analytical and digital preservation point of view. As such, the acquisition of colour-accurate digital images is one of the primary research topics in the international graffiti project INDIGO.
The problem with colour is that it is a matter of perception. In other words: colour is subjective. In addition, the colour data registered by consumer digital cameras (usually in the well-known RGB colour model) are not strictly colourimetric. The camera’s built-in imaging sensor does not satisfy the Luther-Ives condition, which means that its three spectral sensitivity curves (one for Red, one for Green and one for Blue) do not entirely mimic those of the human eye. Moreover, these spectral curves are device-dependent; in other words: they differ from camera to camera. Finally, every camera brand processes the images in their proprietary way to yield a pleasing photograph, not a colour-accurate one.
A digital camera is thus not suitable for rigorous colour determination without any colour correction procedure. Due to all the challenges involved, INDIGO has set itself two aims: 1) to develop a rigorous colourimetric image processing workflow and 2) to implement it into an intuitive, user-friendly and open-source toolbox. This toolbox will provide any user complete control when processing images into colour-accurate products. At the same time, users can also learn about each processing step. We are confident that this research will benefit future graffiti and other heritage documentation projects, as well as any application where digital cameras play a fundamental role in acquiring correct colour values.

Awareness of the 20th-century graffiti phenomenon spread globally with movies like Wild Style (19... more Awareness of the 20th-century graffiti phenomenon spread globally with movies like Wild Style (1983) and photo books like Subway art (1984). To educate the public on the graffiti movement, graffiti specific terms were described and discussed throughout such books or added at the end in the form of a glossary – an alphabetical list of words. However, these glossaries aimed only to explain the graffiti community terms to the broader public rather than to build up a fixed set of terms (and synonyms) or guidelines for the description of graffiti.
This talk will illustrate how the INDIGO project attempts to fill this gap by building a thesaurus, i.e. a structured, controlled (and thus finite) dictionary of hierarchically related terms. The creation of such a thesaurus must tackle particular challenges characteristic of graffiti, including: regional terms that can differ widely (international: stencil; French: pochoir); constantly changing terminology (hit → tag; masterpiece → piece); lack of connection between modern-day graffiti and historical forms of personalised expression like cave art and petroglyphs (studied in archaeology).
These challenges require technical solutions that make it possible to precisely structure the thesaurus items while also describing the relationships between the terms in different ways and reconciling inconsistent terminology. Therefore, this presentation will detail how INDIGO profits from modern Semantic Web technologies. Thanks to these, single pieces of information can be represented according to formally defined models, linked to each other, and made available on the web for reference and reuse by a wider audience.
More specifically, the talk will illustrate the advantages of the model being used by the INDIGO thesaurus, the Simple Knowledge Organisation System (SKOS), a recommendation of the World Wide Web Consortium employed in a wide range of fields, including cultural heritage. This model allows to document the thesaurus terms accurately (e.g. providing historical information about them), describe their hierarchical (and non-hierarchical) relationships, and link them to existing initiatives, such as Wikidata and the Getty Art & Architecture Thesaurus.
The development of the INDIGO thesaurus will allow the project team to have a clear, explicit, and reproducible way to describe graffiti and their constituent parts. At the same time, since the thesaurus will also be the product of the experience and feedback provided by external experts, it is envisaged that it will serve as a reference for the broader (academic) graffiti community.

Graffiti and street art are unique forms of cultural heritage which have not yet received the sci... more Graffiti and street art are unique forms of cultural heritage which have not yet received the scientific attention they deserve. The potential of graffiti research to answer a variety of social and cultural research questions is underexploited. One reason for this lack of academic rigor lies within the very sparse availability of well-curated and publicly accessible graffiti data. One obvious foundation for a graffiti dataset is images depicting the various pieces, tags or throw-ups. The few existing graffiti image databases do not allow the study of the graffiti’s exact shapes and dimensions as the photographs suffer from various distortions which are introduced by topography, perspective, or the camera itself. Orthophotos have the great advantage of removing above-mentioned image distortions. The creation of orthophotos, however, is time-consuming and requires considerable manual interaction, which poses a significant obstacle considering that comprehensive and up-to-date graffiti databases often deal with hundreds of images per week.

Here, we present a novel methodology which overcomes the above-mentioned limitations in present-day graffiti documentation by deriving graffiti orthophotos in a highly automated way. In contrast to conventional photographs, orthophotos are adjusted for the graffiti surface topography and had all perspective and lens distortions removed. As a result, orthophotos allow the measurement of coordinates, distances, and areas of the depicted graffiti, enabling the derivation of exact proportions and relating the object to a real-world geographic coordinate reference system. Orthophotos in this context are therefore accurate maps of graffiti. The derivation of orthophotos, however, is complex and requires not only exact knowledge about the three-dimensional surface geometry of the object but also about the camera’s internal geometry and its exact location and orientation in space. We are developing a photogrammetric methodology and associated tool that derives the required information based on graffiti photos and outputs the desired orthophotos without any manual intervention during the process.

In this talk, we present this bespoke photogrammetric approach which is currently investigated in the framework of the graffiti documentation and dissemination project INDIGO and is built as a Python add-on to the commercial package Agisoft Metashape Professional. First tests were conducted at the Donaukanal (Eng. Danube Canal), a central waterway through Vienna and a famous graffiti hotspot. The results indicate a reliable performance of our approach: the majority (ca. 85%) of the tested graffiti images were successfully and automatically turned into orthophotos. We will present those results and discuss where further finetuning is needed and demonstrate why the presented tool has the potential to largely automize the way graffiti are being documented. This approach will enable the building of a solid and extensive database for advanced graffiti research.

Project INDIGO wants to ensure the digital survival of a large part of Vienna's graffiti-scape by... more Project INDIGO wants to ensure the digital survival of a large part of Vienna's graffiti-scape by creating a long-term, accurate, exhaustive, open access and interactive online archive. The digital photographs of graffiti constitute one of the archive's main backbones. Not only do they represent the spectral and spatial data about every graffito, but they are also used to extract the three-dimensional surface geometry needed to turn these photographs into colour-accurate, distortion-free orthophotographs. That is why INDIGO photographs are taken according to specific rules and with dedicated hardware.
This talk will explain the rationale of INDIGO's photographic approach and detail the entire photo acquisition process: from the logistics for the bi-annual total coverage surveys (targeting the whole research area) to the camera settings and newly developed hardware for the bi-weekly photo tours (aiming to record new graffiti). The presentation will expand on INDIGO's current reliance on human memory and social media combined with specific apps to keep track of must-record changes in the graffiti-scape. However, the talk also reviews plans for a more automated change detection approach.
Overall, this contribution wants to clarify that photographs, whether of graffiti or any other form of cultural heritage, typically serve a specific purpose. To reach that purpose, photographs should usually be more than mere snapshots. Via this talk's focus on INDIGO-specific photography strategies and hardware, the INDIGO team hopes to get critical feedback, valuable suggestions and collaboration proposals. These would benefit everybody involved and ideally lead to the joint development of more dedicated photo acquisition workflows for any cultural heritage recording or monitoring project.

Presentation given by Benjamin Wild at the 3D-ARCH conference and winner of the best paper award.

Keynote at CIPA 2019 Ávila - The 27th CIPA Heritage Documentation international symposium "Documenting the past for a better future", Sep 2, 2019

The epistemological foundation of archaeology and cultural heritage at large is based mainly on t... more The epistemological foundation of archaeology and cultural heritage at large is based mainly on the connections between imperfectly preserved material culture and humans. The tangible part of cultural heritage is thus a lot about materiality, which is reflected in the amalgam of data recording and documentation practices. Whether distinguishing a painting’s anthropogenic stratification created by its layers of overpainting, or spotting buried geo-archaeological marks from an aeroplane, faster, more accurate, more precise, more reliable and more affordable digital techniques are enabling new and better digital approximations of our tangible cultural heritage.

A typical example is the 21st-century hype of using image-based modelling or scanning techniques to acquire three-dimensional (3D) surface geometries. However, the general prioritisation of digital 3D data acquisition over added heritage knowledge results in an undeniable tension between cultural heritage, and the aims and strategies of the Geomatics industry from which it borrows tools and practices. Too often, this 3D datafication wave is fed by the belief that the mere creation of digital surface models will allow scholars all over the world to study heritage in ways previously unheard of. Although partly correct, statements like these distract from the fact that these digital 3D data only represent one of the many essential characters of a heritage asset (such as spectral reflectance, mass, building material), while the creation of a comprehensive narrative does not come overnight either. Archival records, archaeological databases, historical sources and other analytical techniques must supplement these digital geometry data to generate new historical-archaeological-cultural insights.

Moreover, digital surface approximations of cultural heritage assets do not diminish the threats faced by natural disasters and human conflicts. Since most heritage places cannot be expected to last into the coming centuries or even decades due to their fragile nature, their meticulous 3D surface recording by planet-traversing teams will do nothing to protect and conserve them. Although organisations like CyArk would want everybody to believe otherwise, a digital approximation of merely one aspect (i.e. the surface) of a complete building, artefact or place will never bring the original back. Even if it might support digital or physical recreation (thereby unleashing the cultural identity/heritage authenticity debate), the 3D data still have to withstand the digital ravages of time. Accidentally deleted data, data destroyed by fire, data on failed hard drives, data in undocumented formats or simply technology obsolescence; all these issues of digital 3D data longevity are known all too well.

Finally, this 3D datafication industry is still plagued with fundamental issues related to terminology, data protection and ownership, accuracy, reliability, standardisation and perception. Given these numerous critical problems and questions, should one really praise the current 3D datafication hype? Are the countless 3D models actually that transformative for cultural heritage as is often claimed?

By highlighting some of these concerns, this talk wants to question whether the ongoing 3D datafication (r)evolution will stand the test of time or prove to be just a flash in the pan that failed to bring paradigm-shifting, heritage-beneficial developments along. This talk will be provocative and hopes to function as a starting point for a more in-depth debate about this topic.

Verhoeven, G, Štuhec, S., Štuhec, I., 2019. Modelling building costs from 3D building models. 3D-ARCH 2019 "3D Virtual Reconstruction and Visualization of Complex Architectures", 07 February 2019, Bergamo, Italy. DOI: 10.5281/zenodo.2557762, Feb 7, 2019

This is a presentation that was given at Ghent University during the research seminar "Between di... more This is a presentation that was given at Ghent University during the research seminar "Between digital photography and virtual reality in archaeology". It covers the spectral and spatial dimensions of photographs and how these dimensions can be used in archaeological research.

disseminate | analyse | understand graffiti-scapes. Proceedings of the goINDIGO 2023 international graffiti symposium, Aug 6, 2024

A digital photo file contains the image pixel values along with associated photo metadata. Storin... more A digital photo file contains the image pixel values along with associated photo metadata. Storing those metadata is enabled by various standards. For instance, the Exif standard enables the recording of technical photo metadata like the camera's serial number and focal length, while the IPTC Photo Metadata Standard is the widely accepted norm for storing copyright and descriptive information in images (from unedited photos to AI-generated pictures). Since its 2019.1 version, the IPTC Photo Metadata Standard has facilitated the creation of image regions: groupings of image pixels-defined by a circle, rectangle, or any other polygonal shape-which can be annotated with region-specific metadata. Given the potential of image regions for graffiti photo annotation, the open-source and freely available software GRAPHIS was developed within the academic graffiti project INDIGO. GRAPHIS (Generate Regions and Annotations for PHotos using the IPTC Standard) allows users to generate and visualise image regions, annotate them with graffiti descriptions or transcriptions, and save them as metadata within the image. To adhere to the IPTC Photo Metadata Standard at every stage, project INDIGO also created a dedicated controlled vocabulary to contain all relevant concepts that can be used to define each image region's role and content type. This paper starts with a general overview of metadata concepts, followed by a more in-depth look at Exif and IPTC photo metadata. After describing the IPTC Image Region property, the text details the workings of GRAPHIS and the controlled vocabulary development. An overview of use cases and potential software improvements conclude the text.

disseminate | analyse | understand graffiti-scapes. Proceedings of the goINDIGO 2023 international graffiti symposium, Sep 6, 2024

In the year of Hip-Hop’s 50th anniversary and the 40th birthday of Wild Style, goINDIGO 2023 mana... more In the year of Hip-Hop’s 50th anniversary and the 40th birthday of Wild Style, goINDIGO 2023 managed to bring various disciplines together to discuss the various ways of dissipating and understanding bits of graffiti bytes. The proceedings of this symposium pick up and reiterate where the previous proceedings left off: with the contemporary ways of disseminating old or new graffiti. Afterwards, several papers discuss practical and theoretical ways to unravel graffiti-scapes and develop new insights. In summation, tackling graffiti as sociocultural artefacts demands multidisciplinary frameworks. The editors hope the interconnected graffiti themes covered here and in the goINDIGO 2022 proceedings provide inspiration and an up-to-date overview of various framework components to deal with ancient and contemporary graffiti-scapes.

disseminate | analyse | understand graffiti-scapes. Proceedings of the goINDIGO 2023 international graffiti symposium, Sep 6, 2024

Time has remained one of the hardest-to-grasp properties of nature despite humans talking about t... more Time has remained one of the hardest-to-grasp properties of nature despite humans talking about time… all the time. However, even academic fields that are indifferent to the exact physical or philosophical characteristics of time must find ways to engage with the temporal dimension of their data. This applies to all of the Digital Humanities and maybe most to archaeology, a discipline focused on examining space- and time-bound anthropogenic activities. Like archaeological sites and landscapes, graffiti-scapes are spatially and temporally stratified. That is why the academic graffiti project INDIGO uses an archaeological lens to document, disseminate and investigate an urban graffiti-scape in space and time. However, since archaeologists still lack effective practical approaches to manage and visualise the temporal data dimension (besides a handful of data modelling standards and tools, both mainly created by geographers), INDIGO is currently developing graffiti-specific approaches to manage, visualise and analyse the uncertain spatio-temporal boundaries characterising these contemporary artefacts. After a general introduction to time and its relevance for archaeology and the study of graffiti, this paper explains why and how INDIGO uses polygons as digital representations for each real-world graffito. These polygons, stored in a human- and machine-readable file format and annotated with detailed temporal data, aim to provide a nuanced documentation of a graffiti-scape’s spatio-temporal dimensions.

disseminate | analyse | understand graffiti-scapes. Proceedings of the goINDIGO 2023 international graffiti symposium, Sep 6, 2024

This paper presents Urban Chameleon, an online platform currently developed within INDIGO, an aca... more This paper presents Urban Chameleon, an online platform currently developed within INDIGO, an academic project that aims to inventory, disseminate, and analyse contemporary graffiti along the Donaukanal (Eng. Danube Canal) in Vienna, Austria. Urban Chameleon serves as a digital space to visually and analytically explore these graffiti. The paper provides an in-depth overview of the platform's development process, including data modelling underpinnings and Web development technologies. The text further discusses the platform's potential applications for researchers, heritage professionals, graffitists and the wider public interested in engaging with the vibrant and dynamic graffiti-scape along the Danube Canal. In that way, the article contributes to the ongoing discourse on graffiti as a multi-faceted, modern cultural practice with heritage value.

ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Jun 23, 2023

Colourful and ever-changing: Graffiti can be considered the urban chameleon skin. At the Donaukan... more Colourful and ever-changing: Graffiti can be considered the urban chameleon skin. At the Donaukanal (Eng. Danube Channel), Vienna's central waterway and one of the largest and most active graffiti-scapes worldwide, this metaphor applies like hardly anywhere else. Every day a multitude of graffiti is destroyed by the creation of new works. Recently, efforts have been made to mitigate this constant loss of cultural heritage along the Donaukanal by systematically documenting the graffiti, mainly using photography and photogrammetry. However, keeping track of the newly added works is very time-consuming and often like finding needles in a haystack, considering the large extent and high volatility of the monitored area. Thus, an automated graffiti change detection would significantly reduce the effort and avoid overlooking graffiti. This contribution outlines the main challenges in image-based change detection for cultural heritage and proposes a hybrid graffiti change detection method. The investigated method exploits and combines an established pixel-based change detection algorithm, the Iteratively Multivariate Alteration Detection, with a novel descriptor-based method. The latter relies on image features, rather than pixels as analysis unit and can robustly filter false alarms from the high-performing but noise-prone pixel-based approach. Overall, the results indicate that the proposed method can largely automate image-based change detection of graffiti-scapes. It can uncover graffiti-related changes and robustly distinguish them from other image differences such as shadows but tends to overlook small-scale graffiti, indicating the need for further fine-tuning.

La memoria digitale: forme del testo e organizzazione della conoscenza. Atti del XII Convegno Annuale AIUCD (AIUCD 2023), Jun 4, 2023

Graffiti have always been a contested form of human expression. Although they have increasingly b... more Graffiti have always been a contested form of human expression. Although they have increasingly been accepted as museum objects or study topics, in many cases they are neglected or even actively rejected as human expressions worth documenting and (digitally) preserving. However, some scholarly initiatives have tried to overcome this lack of coverage of the graffiti scene. INGRID, for example, collects images from Germany over a span of forty years, while SprayCity archives photos of graffiti from all over Austria and beyond. The limitations with pre-existing solutions, however, is that they do not have standardized criteria to create and process the digital surrogates of graffiti, especially because they derive the photos from different sources. Project INDIGO aims to overcome these difficulties by offering a systematic, standardized and extensive coverage of a specific area of the city of Vienna, the Danube Canal, which has been a hotspot for graffiti production since the 1980s. For this purpose, INDIGO has developed a highly reliable and reproducible workflow that ensures color accuracy and automated orthorectification of the pictures, and has combined it with a curated digital preservation strategy that involves different services, such as the digital repository ARCHE and the spatial database application OpenAtlas. While drawing on the specific experience and knowledge acquired in the course of project INDIGO, this paper aims to offer an overview of the challenges that must be faced when digitizing, documenting and preserving contemporary graffiti. It will focus on three key aspects: (1) heritagization, i.e. the process by which an object made by humans such as a graffito becomes a valuable sample of cultural heritage; (2) FAIRness, the compliance with the so-called FAIR principles (findability, accessibility, interoperability, reusability) in the development of a digital preservation strategy for graffiti; (3) sustainability, i.e. how we can ensure that the data and services provided can integrate into the existing research environment, in order to safeguard the continuous relevance of the products of the research project. While graffiti are an edge case in the field of current studies, this paper aims to show that discussing the challenges linked to the digital preservation of graffiti can also help us think more carefully about possible improvements in digital preservation strategies for other, more traditionally accepted, kinds of cultural heritage assets.

document | archive | disseminate graffiti-scapes. Proceedings of the goINDIGO 2022 international graffiti symposium, May 11, 2023

Graffiti are studied by, amongst many others, archaeologists, sociologists, (art) historians, lin... more Graffiti are studied by, amongst many others, archaeologists, sociologists, (art) historians, linguists, ethnographers, architects, anthropologists, librarian scientists, geographers, criminologists, conservators, lawyers and architects. Although most of these professions rely on a digital representation of graffiti at a particular stage of their research, there has been strikingly little attention to how graffiti can effectively be monitored and digitally documented. And this is precisely one of the gaps that the heritage science project INDIGO is trying to fill. Through collaboration between geomatics, photography, data management and graffiti specialists, INDIGO aims to develop technical and logistical solutions that facilitate the systematic documentation, monitoring, and analysis of extensive graffiti-scapes. This paper focuses on the graffiti-discovering and data acquisition strategies INDIGO has been applying during its first project year. At the same time, the text explores new avenues for improving the existing approaches.

document | archive | disseminate graffiti-scapes. Proceedings of the goINDIGO 2022 international graffiti symposium, May 11, 2023

During the second discussion round of goINDIGO 2022, which took place on Friday, 13 May and was c... more During the second discussion round of goINDIGO 2022, which took place on Friday, 13 May and was called Ethics & legality in graffiti (research), three out of many invited graffiti creators joined a discussion on (potentially provocative) statements with symposium participants (joining in-person and online). The statements, compiled by Geert Verhoeven in consultation with Benjamin Wild and Norbert Pfeifer, were:
* objectivity OVER morals
* objectivity OVER consequences
* graffiti INCLUDES exploitation
* copyright DOES NOT matter
* Donaukanal graffiti IGNORES the origins

document | archive | disseminate graffiti-scapes. Proceedings of the goINDIGO 2022 international graffiti symposium, May 11, 2023

Editorial for the proceedings

document | archive | disseminate graffiti-scapes. Proceedings of the goINDIGO 2022 international graffiti symposium, May 11, 2023

Colour is a powerful communication element in most forms of cultural heritage. This importance of... more Colour is a powerful communication element in most forms of cultural heritage. This importance of colour notwithstanding, the documentation of cultural heritage typically captures the geometrical aspects and seldom the spectral dimensions of an artefact. This is partly because the science of colour (called colorimetry) is non-trivial. In addition, capturing accurate colour data with digital cameras remains challenging due to the operating principle of standard imaging sensors and the need for a stable and well-characterised illumination source. Despite these limitations, the heritage science project INDIGO made it one of its central aims to generate colour-accurate photos from graffiti captured with standard digital cameras in varying outdoor illumination conditions. This paper first discusses the importance of colour accuracy in graffiti documentation. Afterwards, the text details (in a non-mathematical manner) essential colorimetric and camera principles that underlie the generation of colour images from raw image sensor data. This in-depth coverage supports clarifying the main hurdles to accurate photo colours. Finally, the paper introduces the open-source COOLPI software resulting from this research. We are confident that COOLPI will benefit any other heritage documentation project, or any application where digital cameras play a fundamental role in acquiring correct colour values.

document | archive | disseminate graffiti-scapes. Proceedings of the goINDIGO 2022 international graffiti symposium, May 11, 2023

Graffiti are polarising. Some consider them vandalism, others part of our cultural heritage. If w... more Graffiti are polarising. Some consider them vandalism, others part of our cultural heritage. If we consider graffiti to be part of our cultural heritage, we should also treat them as such. However, long-term and detailed graffiti documentation initiatives are sparse, so many of the existing archives with graffiti records are biased and incomplete. In addition, graffiti records usually suffer from decontextualisation, that is the lack of environmental information (be it spatially, temporally, but also smell and weather conditions). This means that graffiti documentation might not reflect the intended setting or meaning by the creator. INDIGO, a graffiti-centred academic project, largely overcomes the issue of decontextualisation by designing and implementing photogrammetric engineering approaches that support the ongoing documentation of an extensive graffiti-scape. The latter is situated along the Donaukanal, Vienna's central waterway and one of the most prominent graffiti hotspots worldwide. One innovation developed in the framework of INDIGO is a freely available Metashape add-on called AUTOGRAF. AUTOGRAF employs photogrammetric computer vision techniques to automatically create ortophotographs from all photographed graffiti. Orthophotographs or orthophotomaps are distortion-free images, combining photographs' visual qualities with characteristics of maps. They allow embedding the graffiti in their native, albeit virtual, 3D environment and can thus largely overcome decontextualisation. In this contribution, we showcase the significant advantages of orthophotomaps over conventional photographs and introduce the AUTOGRAF-based workflow that allows the automated derivation of graffiti orthophotos. INDIGO will use this tailor-made tool to enable graffiti analysis in unprecedented detail by mapping and displaying graffiti in their original setting along the Donaukanal.

document | archive | disseminate graffiti-scapes. Proceedings of the goINDIGO 2022 international graffiti symposium, May 11, 2023

What does it mean to create graffiti? What exactly is (or are) graffiti? Graffiti and street art ... more What does it mean to create graffiti? What exactly is (or are) graffiti? Graffiti and street art differ, right? Almost everyone has an opinion on what constitutes (modern) graffiti. Consequently, the term has taken on the most varied meanings in the conversations of academics, in media coverage, or in daily life. So how can one be sure about the meaning of the term (or any graffiti-related term, for that matter)? This is why glossaries, dictionaries, and other types of lists with definitions exist on websites, at the beginning or end of coffee-table books, and in scientific articles. However, there is currently no generally available, updateable, broadly accepted and easy-to-expand list of graffiti-related terms. Therefore, in order to meet the need for systematisation and consistency required for a more comprehensive study of graffiti, project INDIGO embarked on the journey to create a graffiti thesaurus. Being a finite set of terms (i.e. a controlled vocabulary) with hierarchical relations, this thesaurus will make INDIGO's graffiti classification explicit and hopes to serve as a reference for the broader (academic) graffiti community.

document | archive | disseminate graffiti-scapes. Proceedings of the goINDIGO 2022 international graffiti symposium, May 11, 2023

During the first discussion round of goINDIGO 2022, which took place on Thursday, 12 May and was ... more During the first discussion round of goINDIGO 2022, which took place on Thursday, 12 May and was called Creators vs Academics, four local graffiti creators were invited to consider a series of (potentially provocative) statements in discussion with symposium participants (joining in-person and online). The statements, compiled by Geert Verhoeven in consultation with Samuel Merrill, were:
* Academia CHANGES graffiti
* Graffiti MUST be recorded
* ALL graffiti are archive-worthy
* Decontextualisation MATTERS
* Graffiti NEED categorisation
* Digital media are ESSENTIAL

Advances in On- and Offshore Archaeological Prospection: Proceedings of the 15th International Conference on Archaeological Prospection., 2023

The publication at hand are the proceedings of the 15th International Conference on Archaeologica... more The publication at hand are the proceedings of the 15th International Conference on Archaeological Prospection held between March 28 and April 1, 2023, in Kiel, Germany. The content of the articles ranges from local to large-scale case studies all over the world and from various archaeological times, over methodological improvements, new processing and visualization techniques to a special session on marine and wetland prospection. Thus, the collection of articles summarizes the state of the art of prospection methods for on- and offshore archaeological investigations.

Kurzlebig und doch allgegenwärtig, bewundert und gehasst, legal und illegal: Graffiti sind polari... more Kurzlebig und doch allgegenwärtig, bewundert und gehasst, legal und illegal: Graffiti sind polarisierend. Manche sehen in Graffiti schützenswertes kulturelles Erbe, andere Vandalis-mus. Graffiti sind Teil des öffentlichen Raums und damit Teil unseres öffentlichen Lebens, unserer Gesellschaft. Ganz besonders wird dies an den Wänden des Wiener Donaukanal deutlich, wo die Kreation eines neuen Graffito meist die irreversible Zerstörung eines darun-terliegenden Graffitos bedeutet. So verschwinden täglich viele Graffiti unbemerkt und und-okumentiert vom Donaukanal. INDIGO, ein Graffiti-zentriertes Forschungsprojekt, ver-sucht dem Rechnung zu tragen und schafft die Basis für eine systematische Dokumentati-on und Analyse der 13 km langen Graffiti-Landschaft am Donaukanal. Grundlage der Dokumentation bilden zehntausende Fotos, die im Laufe der 2-jährigen Projektphase auf-genommen, prozessiert und in einem digitalen 3D-Modell des Donaukanals dargestellt werden. Dieser Beitrag gibt einen Überblick über das Projekt INDIGO mit einem Fokus auf die photogrammetrische Auseinandersetzung mit dem Thema Graffiti.

ISPRS Archives, Feb 25, 2022

Image stabilisation (IS) is a family of approaches whose aim is to reduce motion blur in still im... more Image stabilisation (IS) is a family of approaches whose aim is to reduce motion blur in still images and shaking effect in video frames. A variety of techniques are currently implemented in cameras and camcorders: some involve hardware solutions, other are software approaches. In general, IS for still photography entails hardware in-camera or in-lens solutions. Video stabilisation, on the other hand, can be accomplished with software algorithms, either in real-time within the camera or in post-processing. Whereas IS aids photography and video making, its influence on the photogrammetric 3D modelling process has not been investigated. This article addresses this aspect. To this purpose, several laboratory and real-world tests were carried out, whose results showed that IS must be disabled when accuracy matters in photogrammetric projects. Details are provided in the manuscript.

Int. Arch. Photogramm. Remote Sens. Spatial Inf. Sci., Feb 25, 2022

Graffiti is a short-lived form of heritage balancing between tangible and intangible, offensive a... more Graffiti is a short-lived form of heritage balancing between tangible and intangible, offensive and pleasant. Graffiti makes people laugh, wonder, angry, think. These conflicting traits are all present along Vienna's Donaukanal (Eng. Danube Canal), a recreational hotspot – located in the city's heart – famous for its endless display of graffiti. The graffiti-focused heritage science project INDIGO aims to build the basis to systematically document, monitor, and analyse circa 13 km of Donaukanal graffiti in the next decade. The first part of this paper details INDIGO's goals and overarching methodological framework, simultaneously placing it into the broader landscape of graffiti research. The second part of the text concentrates on INDIGO's graffiti documentation activities. Given the project's aim to create a spatially, spectrally, and temporally accurate record of all possible mark-makings attached in (il)legal ways to the public urban surfaces of the Donaukanal, it seems appropriate to provide insights on the photographic plus image-based modelling activities that form the foundation of INDIGO's graffiti recording strategy. The text ends with some envisioned strategies to streamline image acquisition and process the anticipated hundreds of thousands of images.

The surface of most heritage objects holds important clues about their creation. To answer specif... more The surface of most heritage objects holds important clues about their creation. To answer specific research questions about a 16 thcentury mural painting located in the Bischofstor of Vienna's St. Stephen's Cathedral, the three-dimensional (3D) geometry of the entire painted surface was digitised in minuscule detail using thousands of overlapping photographs. Although this article provides image acquisition and processing specifics, it aims to assess which image-based modelling workflow can achieve the most detailed, noise-free, two-and-a-half dimensional (2.5D) raster surface of this mural painting. Other than their full 3D counterparts and in contrast to the focus of most academic research, 2.5D raster surfaces are ideally suited for visualising and analysing sizeable, detailed surfaces. They are, therefore, still the preferred surface encoding of many heritage projects that want to leverage digital surface approximations to further heritage insights (and not just use them as mere eyecatchers). In the end, only a combination of different 2.5D rasters was able to accurately represent the variable surface of this mural painting with the right amount of spatial detail.

New global perspectives on archaeological prospection. Proceedings of the 13th International Conference on Archaeological Prospection, 28 August – 1 September 2019, Sligo - Ireland, 2019

goINDIGO 2022 has managed to bring various disciplines together. That is why the editors hope tha... more goINDIGO 2022 has managed to bring various disciplines together. That is why the editors hope that the contributions in these proceedings can collectively be considered a proper methodological status quo on the inventorying and dissemination of graffiti records. Because most academic efforts focus on the analyses of graffiti, these proceedings also hope to kickstart further discussion and interdisciplinary scholarly action on the (need for) proper documentation and dissemination of graffiti. Critical, maybe even uncomfortable, reflections like those vented in this volume form an essential part of this discourse.