function dist_km = gcircle_fn(data)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% USAGE:
%
% gcircle_fn([ lon1 lat1;  lon2 lat2 ])
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% GCIRCLE v1.2 by Hamish Bowman  June 7, 2000
% The sphere geodesic (great circle) to calculate the shortest distance betweem two
% points on a shere. See http://mathworld.wolfram.com/GreatCircle.html
%
% This code is gifted to the public domain.
%
% Conventions:
%  degrees
% N lat = + lat
% S lat = - lat
% E lon = lon
% W lon = 360 - lon
%
% data = [ lon1 lat1; lon2 lat2 ]
% ====== 2x2 matrix, column one is lons, 2 is lats. ======
% data = [ 287   42.0;  168  -45.6];    % NY to NZ
%	    


if( ~exist('data'))	% note: need to pass gcircle_fn(data), not just in who!!
    disp('No data found. Using example as default.')
    disp('data = [ lon1 lat1; lon2 lat2 ];')
    break
end


%lon (western hemisphere is 360 - lon!)
lam1 = data(1,1) * (2*pi/360);	% convert to radians
lam2 = data(2,1) * (2*pi/360);

%lat
del1 = data(1,2) * (2*pi/360);
del2 = data(2,2) * (2*pi/360);

r = 6367.444;    % mean radius of the earth (km)

R1dotR2 = cos(del1) * cos(del2) * cos(lam1-lam2) + sin(del1) * sin(del2);

dist_km = r * acos( R1dotR2 );