[Proj] Spheroidal gnomonic projection

[Karney, Charles]

> From: Clifford J Mugnier [cjmce at lsu.edu]
> Sent: Tuesday, June 01, 2010 14:18
> 
> Cole did the two-point azimuthal equidistant on the Helmert ellipsoid
> back in the early 1900's for the Survey of Egypt.  I do not have a
> copy, though.  It's pretty obscure, but the intent was to provide a
> graphic for determining the direction to face for daily prayers.

Thanks, this gives me something to track down.  I assume that "two-point
azimuthal equidistant" means the azimuth from basepoint A is correct and
the distance from basepoint B is correct?  I had thought that Close (and
not Cole) was the originator of this projection:

  C. F. Close, 1935, Two-Point Azimuthal-Equidistant projection,
  Geographical Journal, v. 86, no. 5, p. 445-446.

But maybe Cole generalizes this the the ellipsoid?

The gnomonic projection is the limit of a "two-point azimuthal"
projection where the two azimuths from A and B are correct, and this was
given (for the sphere only?) by Maurer, Immler, and Close (again).  All
of these are in papers I haven't been able to get hold of yet.

  --Charles
--
Charles Karney <ckarney at sarnoff.com>
Sarnoff Corporation, Princeton, NJ 08543-5300

Tel: +1 609 734 2312
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