Geodesics on an ellipsoid — geographiclib 2.0 documentation (original) (raw)

Introduction¶

Consider an ellipsoid of revolution with equatorial radius a, polar semi-axis b, and flattening f = (a − b)/a . Points on the surface of the ellipsoid are characterized by their latitude φ and longitude λ. (Note that latitude here means the_geographical latitude_, the angle between the normal to the ellipsoid and the equatorial plane).

The shortest path between two points on the ellipsoid at (φ1, λ1) and (φ2, λ2) is called the geodesic. Its length is_s_12 and the geodesic from point 1 to point 2 has forward azimuths α1 and α2 at the two end points. In this figure, we have λ12 = λ2 − λ1.

A geodesic can be extended indefinitely by requiring that any sufficiently small segment is a shortest path; geodesics are also the straightest curves on the surface.

Solution of geodesic problems¶

Traditionally two geodesic problems are considered:

• the direct problem — given φ1, λ1, α1, _s_12, determine φ2, λ2, and α2; this is solved byGeodesic.Direct.
• the inverse problem — given φ1, λ1, φ2, λ2, determine _s_12, α1, and α2; this is solved byGeodesic.Inverse.

Additional properties¶

The routines also calculate several other quantities of interest

• _S_12 is the area between the geodesic from point 1 to point 2 and the equator; i.e., it is the area, measured counter-clockwise, of the quadrilateral with corners (φ1,λ1), (0,λ1), (0,λ2), and (φ2,λ2). It is given in meters2.
• _m_12, the reduced length of the geodesic is defined such that if the initial azimuth is perturbed by _d_α1(radians) then the second point is displaced by _m_12 _d_α1 in the direction perpendicular to the geodesic. _m_12 is given in meters. On a curved surface the reduced length obeys a symmetry relation, _m_12 +_m_21 = 0. On a flat surface, we have _m_12 =_s_12.
• _M_12 and M_21 are geodesic scales. If two geodesics are parallel at point 1 and separated by a small distance_dt, then they are separated by a distance _M_12 dt at point 2. _M_21 is defined similarly (with the geodesics being parallel to one another at point 2). _M_12 and_M_21 are dimensionless quantities. On a flat surface, we have _M_12 = _M_21 = 1.
• σ12 is the arc length on the auxiliary sphere. This is a construct for converting the problem to one in spherical trigonometry. The spherical arc length from one equator crossing to the next is always 180°.

If points 1, 2, and 3 lie on a single geodesic, then the following addition rules hold:

• _s_13 = _s_12 + _s_23
• σ13 = σ12 + σ23
• _S_13 = _S_12 + _S_23
• _m_13 = _m_12_M_23 +_m_23_M_21
• _M_13 = _M_12_M_23 − (1 − _M_12_M_21)_m_23/_m_12
• _M_31 = _M_32_M_21 − (1 − _M_23_M_32)_m_12/_m_23

Multiple shortest geodesics¶

The shortest distance found by solving the inverse problem is (obviously) uniquely defined. However, in a few special cases there are multiple azimuths which yield the same shortest distance. Here is a catalog of those cases:

• φ1 = −φ2 (with neither point at a pole). If α1 = α2, the geodesic is unique. Otherwise there are two geodesics and the second one is obtained by setting [α1,α2] ← [α2,α1], [_M_12,_M_21] ← [_M_21,_M_12], _S_12 ← −_S_12. (This occurs when the longitude difference is near ±180° for oblate ellipsoids.)
• λ2 = λ1 ± 180° (with neither point at a pole). If α1 = 0° or ±180°, the geodesic is unique. Otherwise there are two geodesics and the second one is obtained by setting [α1,α2] ← [−α1,−α2],_S_12 ← −_S_12. (This occurs when φ2 is near −φ1 for prolate ellipsoids.)
• Points 1 and 2 at opposite poles. There are infinitely many geodesics which can be generated by setting [α1,α2] ← [α1,α2] + [δ,−δ], for arbitrary δ. (For spheres, this prescription applies when points 1 and 2 are antipodal.)
• _s_12 = 0 (coincident points). There are infinitely many geodesics which can be generated by setting [α1,α2] ← [α1,α2] + [δ,δ], for arbitrary δ.

Area of a polygon¶

The area of a geodesic polygon can be determined by summing _S_12for successive edges of the polygon (S_12 is negated so that clockwise traversal of a polygon gives a positive area). However, if the polygon encircles a pole, the sum must be adjusted by ±_A/2, where A is the area of the full ellipsoid, with the sign chosen to place the result in (-A/2, A/2].

Background¶

The algorithms implemented by this package are given in Karney (2013) and are based on Bessel (1825) and Helmert (1880); the algorithm for areas is based on Danielsen (1989). These improve on the work of Vincenty (1975) in the following respects:

• The results are accurate to round-off for terrestrial ellipsoids (the error in the distance is less than 15 nanometers, compared to 0.1 mm for Vincenty).
• The solution of the inverse problem is always found. (Vincenty’s method fails to converge for nearly antipodal points.)
• The routines calculate differential and integral properties of a geodesic. This allows, for example, the area of a geodesic polygon to be computed.

References¶

• F. W. Bessel,The calculation of longitude and latitude from geodesic measurements (1825), Astron. Nachr. 331(8), 852–861 (2010), translated by C. F. F. Karney and R. E. Deakin.
• F. R. Helmert,Mathematical and Physical Theories of Higher Geodesy, Vol 1, (Teubner, Leipzig, 1880), Chaps. 5–7.
• T. Vincenty,Direct and inverse solutions of geodesics on the ellipsoid with application of nested equations, Survey Review 23(176), 88–93 (1975).
• J. Danielsen,The area under the geodesic, Survey Review 30(232), 61–66 (1989).
• C. F. F. Karney,Algorithms for geodesics, J. Geodesy 87(1) 43–55 (2013);addenda.
• C. F. F. Karney,Geodesics on an ellipsoid of revolution, Feb. 2011;errata.
• A geodesic bibliography.
• The wikipedia page,Geodesics on an ellipsoid.