function geoddoc
%GEODDOC Geodesics on an ellipsoid of revolution
%
% The routines geoddistance, geodreckon, and geodarea solve various
% problems involving geodesics on the surface of an ellipsoid of
% revolution. These are based on the paper
%
% C. F. F. Karney, Algorithms for geodesics,
% J. Geodesy 87, 43-55 (2013);
% https://dx.doi.org/10.1007/s00190-012-0578-z
% Addenda: http://geographiclib.sf.net/geod-addenda.html
%
% which, in turn, is based on the classic solution of the geodesic
% problems pioneered by Legendre (1806), Bessel (1825), and Helmert
% (1880). Links for these and other original papers on geodesics are
% given in
%
% http://geographiclib.sf.net/geodesic-papers/biblio.html
%
% The shortest path between two points on the ellipsoid at (lat1, lon1)
% and (lat2, lon2) is called the geodesic. Its length is s12 and the
% geodesic from point 1 to point 2 has forward azimuths azi1 and azi2 at
% the two end points.
%
% Traditionally two geodesic problems are considered:
% * the direct problem -- given lat1, lon1, s12, and azi1, determine
% lat2, lon2, and azi2. This is solved by geodreckon.
% * the inverse problem -- given lat1, lon1, lat2, lon2, determine s12,
% azi1, and azi2. This is solved by geoddistance.
% In addition, geodarea computes the area of an ellipsoidal polygon
% where the edges are defined as shortest geodesics.
%
% The parameters of the ellipsoid are specified by the optional ellipsoid
% argument to the routines. This is a two-element vector of the form
% [a,e], where a is the equatorial radius, e is the eccentricity e =
% sqrt(a^2-b^2)/a, and b is the polar semi-axis. Typically, a and b are
% measured in meters and the linear and area quantities returned by the
% routines are then in meters and meters^2. However, other units can be
% employed. If ELLIPSOID is omitted, then the WGS84 ellipsoid (more
% precisely, the value returned by defaultellipsoid) is assumed [6378137,
% 0.0818191908426215] corresponding to a = 6378137 meters and a
% flattening f = (a-b)/a = 1/298.257223563. The flattening and
% eccentricity are related by
%
% e = sqrt(f * (2 - f))
% f = e^2 / (1 + sqrt(1 - e^2))
%
% (The functions ecc2flat and flat2ecc implement these conversions.) For
% a sphere, set e = 0; for a prolate ellipsoid (b > a), specify e as a
% pure imaginary number.
%
% All angles (latitude, longitude, azimuth) are measured in degrees with
% latitudes increasing northwards, longitudes increasing eastwards, and
% azimuths measured clockwise from north. For a point at a pole, the
% azimuth is defined by keeping the longitude fixed, writing lat =
% +/-(90-eps), and taking the limit eps -> 0+.
%
% The routines also calculate several other quantities of interest
% * S12 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 (lat1,lon1), (0,lon1), (0,lon2), and
% (lat2,lon2). It is given in meters^2.
% * m12, the reduced length of the geodesic is defined such that if the
% initial azimuth is perturbed by dazi1 (radians) then the second
% point is displaced by m12 dazi1 in the direction perpendicular to
% the geodesic. m12 is given in meters. On a curved surface the
% reduced length obeys a symmetry relation, m12 + m21 = 0. On a flat
% surface, we have m12 = s12.
% * M12 and M21 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 M12 dt at point 2. M21 is defined
% similarly (with the geodesics being parallel to one another at
% point 2). M12 and M21 are dimensionless quantities. On a flat
% surface, we have M12 = M21 = 1.
% * a12 is the arc length on the auxiliary sphere. This is a construct
% for converting the problem to one in spherical trigonometry. a12
% is measured in degrees. The spherical arc length from one equator
% crossing to the next is always 180 degrees.
%
% If points 1, 2, and 3 lie on a single geodesic, then the following
% addition rules hold:
% * s13 = s12 + s23
% * a13 = a12 + a23
% * S13 = S12 + S23
% * m13 = m12*m23 + m23*m21
% * M13 = M12*M23 - (1 - M12*M21) * m23/m12
% * M31 = M32*M21 - (1 - M23*M32) * m12/m23
%
% Restrictions on the inputs:
% * All latitudes must lie in [-90, 90].
% * All longitudes and azimuths must lie in [-540, 540). On output,
% these quantities lie in [-180, 180).
% * The distance s12 is unrestricted. This allows geodesics to wrap
% around the ellipsoid. Such geodesics are no longer shortest paths.
% However they retain the property that they are the straightest
% curves on the surface.
% * Similarly, the spherical arc length, a12, is unrestricted.
% * The equatorial radius, a, must be positive.
% * The eccentricity, e, should be satisfy abs(e) < 0.2 in order to
% retain full accuracy (this corresponds to flattenings satisfying
% abs(f) <= 1/50, approximately). This condition holds for most
% applications in geodesy.
%
% Larger values of e can be used with a corresponding drop in accuracy.
% The following table gives the approximate maximum error in
% geoddistance and geodreckon (expressed as a distance) for an ellipsoid
% with the same major radius as the WGS84 ellipsoid and different values
% of the flattening (nm means nanometer and um means micrometer).
%
% |f| error
% 0.01 25 nm
% 0.02 30 nm
% 0.05 10 um
% 0.1 1.5 mm
% 0.2 300 mm
%
% The shortest distance returned by GEODDISTANCE 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:
% * lat1 = -lat2 (with neither point at a pole). If azi1 = azi2, the
% geodesic is unique. Otherwise there are two geodesics and the
% second one is obtained by setting [azi1,azi2] = [azi2,azi1],
% [M12,M21] = [M21,M12], S12 = -S12. (This occurs when the longitude
% difference is near +/-180 for oblate ellipsoids.)
% * lon2 = lon1 +/- 180 (with neither point at a pole). If azi1 = 0 or
% +/-180, the geodesic is unique. Otherwise there are two geodesics
% and the second one is obtained by setting [azi1,azi2] =
% [-azi1,-azi2], S12 = -S12. (This occurs when lat2 is near -lat1
% for prolate ellipsoids.)
% * Points 1 and 2 at opposite poles. There are infinitely many
% geodesics which can be generated by setting [azi1,azi2] =
% [azi1,azi2] + [d,-d], for arbitrary d. (For spheres, this
% prescription applies when points 1 and 2 are antipodal.)
% * s12 = 0 (coincident points). There are infinitely many geodesics
% which can be generated by setting [azi1,azi2] = [azi1,azi2] +
% [d,d], for arbitrary d.
%
% In order to compute intermediate points on a geodesic, proceed as in
% the following example which plots the track from JFK Airport to
% Singapore Changi Airport.
%
% lat1 = 40.64; lon1 = -73.78;
% lat2 = 1.36; lon2 = 103.99;
% [s12, azi1] = geoddistance(lat1, lon1, lat2, lon2);
% [lats, lons] = geodreckon(lat1, lon1, s12 * [0:100]/100, azi1);
% plot(lons, lats);
%
% The restriction on e above arises because the formulation is in terms
% of series expansions in e^2. The exact solutions (valid for any e) can
% be expressed in terms of elliptic integrals. These are provided by the
% C++ classes GeodesicExact and GeodesicLineExact.
%
% The routines duplicate some of the functionality of the distance,
% reckon, and areaint functions in the MATLAB mapping toolbox. The major
% improvements offered by geoddistance, geodreckon, and geodarea are
%
% * The routines are accurate to round off for abs(e) < 0.2. For
% example, for the WGS84 ellipsoid, the error in the distance
% returned by geoddistance is less then 15 nanometers.
% * The routines work for prolate (as well as oblate) ellipsoids.
% * geoddistance converges for all inputs.
% * Differential and integral properties of the geodesics are computed.
% * geodarea is accurate regardless of the length of the edges of the
% polygon.
%
% See also GEODDISTANCE, GEODRECKON, GEODAREA,
% DEFAULTELLIPSOID, ECC2FLAT, FLAT2ECC,
% GEODESICDIRECT, GEODESICLINE, GEODESICINVERSE, POLYGONAREA.
% Copyright (c) Charles Karney (2012-2015) <charles@karney.com>.
%
% This file was distributed with GeographicLib 1.43.
help geoddoc
end