ncread - Read data from variable in netCDF data source - MATLAB (original) (raw)

Read data from variable in netCDF data source

Syntax

Description

[vardata](#mw%5F560bd1d3-2f9b-43ab-b80a-ca774a3213e5) = ncread([source](#bsu%5Fi8a-1%5Fsep%5Fmw%5Fe339e1a9-285c-4606-9ca3-5d590980a4d2),[varname](#bsu%5Fi8a-1%5Fsep%5Fmw%5F936cf078-b38a-48b9-bb9f-aff8ed8f14fc)) reads all the data from the netCDF variable varname contained insource. The returned variable value vardata is of the MATLAB® data type that best matches the netCDF data type ofvarname.

example

[vardata](#mw%5F560bd1d3-2f9b-43ab-b80a-ca774a3213e5) = ncread([source](#bsu%5Fi8a-1%5Fsep%5Fmw%5Fe339e1a9-285c-4606-9ca3-5d590980a4d2),[varname](#bsu%5Fi8a-1%5Fsep%5Fmw%5F936cf078-b38a-48b9-bb9f-aff8ed8f14fc),[start](#mw%5Fc40440f3-d75e-421a-a505-30f047a4054a),[count](#mw%5F5d90db5a-9e5c-49a9-a145-a94ee23fd848)) reads data beginning at the location specified by start. Thecount argument specifies the number of elements to read along each dimension.

example

[vardata](#mw%5F560bd1d3-2f9b-43ab-b80a-ca774a3213e5) = ncread([source](#bsu%5Fi8a-1%5Fsep%5Fmw%5Fe339e1a9-285c-4606-9ca3-5d590980a4d2),[varname](#bsu%5Fi8a-1%5Fsep%5Fmw%5F936cf078-b38a-48b9-bb9f-aff8ed8f14fc),[start](#mw%5Fc40440f3-d75e-421a-a505-30f047a4054a),[count](#mw%5F5d90db5a-9e5c-49a9-a145-a94ee23fd848),[stride](#mw%5Fd9c1dcb3-b394-4d44-af97-8bbcb933ebde)) returns data with the interval specified by stride between the indices of each dimension of the variable.

example

Examples

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Read and plot a variable named peaks from the netCDF file example.nc.

peaksData = ncread("example.nc","peaks"); whos peaksData

Name Size Bytes Class Attributes

peaksData 50x50 5000 int16

Plot peaksData and add a title.

surf(peaksData) title("Peaks Data")

Read and plot only a subset of the peaks variable data starting from the location [25 17] until the end of each dimension.

start = [25 17]; % Start location along each coordinate count = [Inf Inf]; % Read until the end of each dimension peaksData = ncread("example.nc","peaks",start,count); whos peaksData

Name Size Bytes Class Attributes

peaksData 26x34 1768 int16

Plot the data.

surf(peaksData) title("Peaks Data Starting at [25 17]")

Read and plot data, where the data is sampled at a specified spacing between variable indices along each dimension. Read variable data at intervals specified in stride, starting from the location in start. A value of 1 in stride accesses adjacent values in the corresponding dimension, a value of 2 accesses every other value in the corresponding dimension, and so on.

start = [1 1]; count = [10 15]; stride = [2 3]; sampledPeaksData = ncread("example.nc","peaks",start,count,stride); whos sampledPeaksData

Name Size Bytes Class Attributes

sampledPeaksData 10x15 300 int16

Plot the data.

surf(sampledPeaksData) title("Peaks Data Subsampled by [2 3]")

Input Arguments

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Name of the netCDF data source, specified as a string scalar or character vector. Thesource argument can be one of these values:

• The path of a local netCDF source
• The OPeNDAP URL of a remote OPeNDAP netCDF data source
• The HTTP URL of a remote netCDF source, with #mode=bytes appended to the end of the URL to enable byte-range reading
  Note
  Byte-range reading is slower than other methods for reading from remote sources. For more details about byte-range reading, see the netCDF documentation.

Example: "myNetCDFfile.nc"

Example: "http://_`hostname`_/_`netcdffilename`_#mode=bytes"

Variable name, specified as a string scalar or character vector containing the name of a variable in the netCDF data source.

If source specifies a file with formatnetcdf4, you can specify the location of the variable within the group hierarchy by specifying varname as a fully qualified name.

Example: "myVar"

Example: "/myGrp/mySubGrp/myNestedVar"

Starting location of the data in the variable, specified as a numeric vector of positive integers. For an _N_-dimensional variable, specifystart as a vector of length N containing 1-based indices.

If you do not specify start, then thencread function starts reading the variable from the first index along each dimension.

Example: [2 1 3]

Data Types: double

Number of elements to read, specified as a numeric vector of positive integers orInf values. For an N-dimensional variable, specify count as a vector of lengthN, containing the number of elements to read along each dimension. If any element of count is Inf, thenncread reads until the end of the corresponding dimension.

If you do not specify count, then thencread function reads the variable data until the end of each dimension.

Example: [Inf 10 50]

Data Types: double

Space between the variable indices along each dimension, specified as a numeric vector of integers. For an _N_-dimensional variable, specifystride as a vector of length N. The elements of the stride vector correspond, in order, to the dimensions of the variable. A value of 1 accesses adjacent values of the netCDF variable in the corresponding dimension, a value of 2 accesses every other value in the corresponding dimension, and so on.

If you do not specify stride, then thencread function reads the data with a default spacing of1 along each dimension.

Example: [2 10 1]

Data Types: double

Output Arguments

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Variable data, returned as a numeric array, text, or cell array of the MATLAB data type that best matches the netCDF data type ofvarname. For more information about how MATLAB determines the best match, see NetCDF to MATLAB Data Type Conversion.

For numeric variables, when at least one of the attributes_FillValue, scale_factor, oradd_offset is present, vardata is of typedouble. Additionally, ncread applies these attribute conventions in a sequence before returningvardata:

1. If the _FillValue attribute exists:
   • If vardata is of type double orsingle, then ncread replacesvardata values equal to that of the_FillValue attribute with NaN values.
   • If vardata is of any other numeric type, thenncread replaces NaN vardata values as well as vardata values equal to that of the _FillValue attribute with0 values.
2. If the scale_factor attribute exists, thenncread multiplies the values in vardata by the value of the scale_factor attribute.
3. If the add_offset attribute exists, thenncread adds the value of the add_offset attribute to the values in vardata.

Note

If varname is of type NC_STRING, then it can contain UTF-8-encoded characters; if varname is of typeNC_CHAR, then it must contain only ASCII-encoded characters.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | cell | char | string

More About

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The netCDF-related MATLAB functions automatically choose the MATLAB data type that best matches the netCDF data type according to this table.

(*) These netCDF data types are available only for files with formatnetcdf4.

Tips

• MATLAB interprets data as column major, but the netCDF C API interprets data as row major. Multidimensional data in the netCDF C API shows dimensions in the reverse of the order shown by MATLAB and consequently appears transposed.

Version History

Introduced in R2011a

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You can use ncread for read-only access to remote datasets using the HTTP byte-range capability, provided that the remote server supports byte-range access.

You can read variable length array data types (NC_VLEN) from netCDF-4 files.

You can read NC_STRING data from netCDF-4 files.