[Proj] Earth gravity (and magnetic) models in GeographicLib

[Charles Karney]

I've added earth gravity models to GeographicLib.  This allows you to
compute quantities like the gravitational field, the deflections of the
vertical, and the geoid height directly from the EGM2008 (or EGM96 or
EGM84) model coefficients.  For details see

  http://geographiclib.sourceforge.net/html/gravity.html

My development of this capability in GeographicLib relied, in part, on
the code for EGM_Geoid_CalculatorClass by Mathieu Peyréga.

GeographicLib is able to reproduce accurately the gridded data for
EGM2008 published by the NGA.  Single point calculations for EGM2008 are
rather slow (78 ms for the geoid height).  However the computations can
be sped up by a factor of 800 by computing several points on a circle of
latitude and by using, for example, OpenMP, for parallel processing.
Thus, on my 8-processor desktop machine I can compute the global 1' grid
of geoid heights in 40 mins (i.e., about 10 us per point).

P.S. Magnetic models are also supported, see

  http://geographiclib.sourceforge.net/html/magnetic.html

-- 
Charles Karney <charles.karney at sri.com>
SRI International, Princeton, NJ 08543-5300

Tel: +1 609 734 2312
Fax: +1 609 734 2662