[Proj] Distance calculations

[Irwin Scollar]

Karl Swartz wrote:

How does this compare to Thomas Vincenty's algorithm (which can be found
at http://www.ngs.noaa.gov/PUBS_LIB/inverse.pdf)?

and Gerald Evenden wrote:

I second the request as I considered it the gold standard and besides full
documentation and source code are available at NGS as well as online
computational services.

An on-line source with a derivation of the Zhang Xue-Lian distance 
calculations can be found in

http://kom.aau.dk/~borre/masters/frames/notes.pdf

pages 18 ff.

I have not found  on-line sources for the Pittman paper or critiques 
noted by Cliff Mugnier.

It would indeed be helpful if the three methods can be compared for accuracy.

Irwin Scollar