function a = deltalam(lam1,lam2,phi)
%
% The function a = deltalam(lam1,lam2,phi) gives as output the
% distance (in km) of two meridians at a latitude phi (assuming the 
% earth is a sphere).  lam1, lam2 should be the longitude of each
% meridian, and phi should be their latitude, in decimal degrees.
%
%---------------------------------------------------------------------
%  Written by Vassilis Zervakis, in June 1996,
%  while at AINCO-Interocean, Madrid, Spain
%---------------------------------------------------------------------

% Let R be the ratio of a sphere with a volume equal to that of the Earth.
R = 6371.;
dlam = lam1 - lam2;
a = (2*pi*R/360.)*dlam*cos(phi*pi/180.);
return
