function [lat, lon, azi, rk] = eqdazim_inv(lat0, lon0, x, y, ellipsoid)
%EQDAZIM_INV Inverse azimuthal equidistant projection
% [LAT, LON] = EQDAZIM_INV(LAT0, LON0, X, Y)
% [LAT, LON, AZI, RK] = EQDAZIM_INV(LAT0, LON0, X, Y, ELLIPSOID)
% performs the inverse azimuthal equidistant projection of points (X,Y)
% to (LAT,LON) using (LAT0,LON0) as the center of projection. These
% input arguments can be scalars or arrays of equal size. The ELLIPSOID
% vector is of the form [a, e], where a is the equatorial radius in
% meters, e is the eccentricity. If ellipsoid is omitted, the WGS84
% ellipsoid (more precisely, the value returned by DEFAULTELLIPSOID) is
% used. GEODPROJ defines the projection and gives the restrictions on
% the allowed ranges of the arguments. The forward projection is given
% by EQDAZIM_FWD.
% AZI and RK give metric properties of the projection at (LAT,LON); AZI
% is the azimuth of the geodesic from the center of projection and RK is
% the reciprocal of the azimuthal scale. The scale in the radial
% direction is 1.
% LAT0, LON0, LAT, LON, AZI are in degrees. The projected coordinates X,
% Y are in meters (more precisely the units used for the equatorial
% radius). RK is dimensionless.
% Section 14 of
% C. F. F. Karney, Geodesics on an ellipsoid of revolution (2011),
% Errata: http://geographiclib.sf.net/geod-addenda.html#geod-errata
% describes how to use this projection in the determination of maritime
% boundaries (finding the median line).
% This routine depends on the MATLAB File Exchange package "Geodesics on
% an ellipsoid of revolution":
% See also GEODPROJ, EQDAZIM_FWD, GEODRECKON, DEFAULTELLIPSOID.
% Copyright (c) Charles Karney (2012) <email@example.com>.
% This file was distributed with GeographicLib 1.29.
if nargin < 4, error('Too few input arguments'), end
if nargin < 5, ellipsoid = defaultellipsoid; end
[~] = lat0 + lon0 + x + y;
error('lat0, lon0, x, y have incompatible sizes')
azi0 = atan2(x, y) / (pi/180);
s = hypot(x, y);
[lat, lon, azi, ~, m, ~, ~, sig] = geodreckon(lat0, lon0, s, azi0, ellipsoid);
rk = m ./ s;
rk(sig <= 0.01 * sqrt(realmin)) = 1;