[GRASS-dev] [Proj] Improvements to geod and invgeod

Helmut Kudrnovsky hellik at web.de
Sun May 26 01:00:16 PDT 2013


taken from http://lists.maptools.org/pipermail/proj/2013-May/006602.html

geod and invgeod, programs for performing geodesic calculations, have
been part of proj.4 since at least 1995.  There are several problems
with them:

* the overall accuracy is limited;

* the results of inverse calculations are sometimes wildly wrong;

* there's no documented API for the geodesic capabilities;

* there's no provision from computing various quantities associated with
   geodesics (in particular the area of a geodesic polygon).

These problems will be addressed in the next release of proj.4 with the
inclusion of the algorithms for geodesics from GeographicLib.  These are
documented in

   C. F. F. Karney,
   Algorithms for geodesics,
   J. Geodesy 87(1), 43-55 (Jan. 2013)
   Addenda: http://geographiclib.sf.net/geod-addenda.html

Points to note:

* Several libraries that depend on proj.4, e.g., GRASS, use approximate
   methods for computing polygonal areas.  They can now use proj.4 to
   compute areas accurately.

best regards
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