[Proj] Azimuthal Equidistant question

Mikael Rittri Mikael.Rittri at carmenta.com
Sat Sep 29 16:16:12 EST 2012

Hello Joseph. 

I haven't found any documentation of how Proj.4 computes aeqd on an ellipsoid.
But the source code is clearly from John P. Snyder, Map Projections: A Working Manual.

(For examle, the assignment 

 c = P->N1 * s * (1. + s * s * (- H2 * ...

 is Equation (25-25) on page 199 of Snyder.)

So Snyder's warnings about the formulas are relevant:

   "These are considered suitable for lines up to 800 km in length."

Because they are intended for local use in Micronesia, not global use. 
But your test points are about 12000 km from the central point, so it seems
that the ellipsoid formulas for aeqd in Proj.4 don't work that far away. 

Disclaimer: I have a fairly old version of Proj.4 on my home computer, but I don't think the aeqd code has been changed recently. 

Best regards,

Mikael Rittri

28 sep 2012 kl. 15:35 skrev "Joseph White" <JWhite at terragotech.com>:

[ text deleted, see http://lists.maptools.org/pipermail/proj/2012-September/006417.html ]

More information about the Proj mailing list