[Proj] Azimuthal Equidistant question

[Mikael Rittri]

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.
http://pubs.er.usgs.gov/publication/pp1395

(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
Carmenta
Sweden
http://www.carmenta.com

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

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