[Proj] wgs72 ellipsoid coeffs?

Hamish hamish_b at yahoo.com
Mon Oct 13 02:04:33 EDT 2008


I wonder if I could tap the braintrust for a moment,

We are trying to verify a great circle distance calculation but I am
stuck on finding a reference for the formula.

Given in the code is:    (MBSystem's mbio/mb_coor_scale.c; GPL)

/* ellipsoid coefficients from World Geodetic System Ellipsoid of 1972 
 * - see Bowditch (H.O. 9 -- American Practical Navigator). */
#define C1 111412.84
#define C2 -93.5
#define C3 0.118
#define C4 111132.92
#define C5 -559.82
#define C6 1.175
#define C7 0.0023


radlat = DTR*latitude;    // degree2radians
*mtodeglon = 1./fabs(C1*cos(radlat) + C2*cos(3*radlat) 
              + C3*cos(5*radlat));
*mtodeglat = 1./fabs(C4 + C5*cos(2*radlat) 
              + C6*cos(4*radlat) + C7*cos(6*radlat));

[...] then given those multipliers the pythagorean theorem is used to
obtain distance for small values of dx,dy.

should C7 be negative?

I have obtained a recent edition of Bowditch's American Practical
Navigator from  http://www.nga.mil/portal/site/maritime/
but searching through the document I couldn't find that. (Ch 24 deals
with great circle distance calcs via table; close but no cigar)

with pen and paper and an old geometry textbook I could only figure
out how to reproduce C4.

Also, if I can find/derive the coefficients, I'd like to update WGS72
->84 to match the gps. (even though the practical difference is probably
nil it is nice to be consistent)

note that MBSys links against proj, so internal libproj functions
could be available for this if there's an easier way.


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