warmerdam at pobox.com
Fri Nov 17 10:43:59 EST 2000
OSSIM, PROJ.4 and Geotiff folks,
I am having a problem getting results of the VanDerGrinten projection in
PROJ.4 to match the results from OSSIM (with projections code derived from
Geotrans from NIMA).
Projection definition (in OSSIM terms):
OSSIM's GrintenProjection results:
Upper Left: lat: 33 54.22N lon: 117 38.47W
Lower Right: lat: 33 37.51N lon: 117 18.48W
Results using PROJ.4:
Upper Left ( -17819.436, 3914081.301) (117d38'27.48"W, 33d52'5.27"N)
Lower Left ( -17819.436, 3879401.301) (117d38'27.06"W, 33d35'23.38"N)
Upper Right ( 17820.564, 3914081.301) (117d18'29.19"W, 33d52'5.27"N)
Lower Right ( 17820.564, 3879401.301) (117d18'29.61"W, 33d35'23.38"N)
Center ( 0.564, 3896741.301) (117d28'28.34"W, 33d43'44.59"N)
The PROJ.4 and OSSIM results for the top left corner are different by
roughly 2 minutes in latitude ... far exceeding what might be expected
from simple numerical errors. Interestingly the OSSIM results very closely
match results from EASI/PACE which uses code derived from the FORTRAN GCTP
Looking into the OSSIM/Geotrans code, I see a "spherical radius" is computed
from the major and minor ellipsoid axes, and is used later, using this
Ra = a * (1.0 - es2 / 6.0 - 17.0 * es4 / 360.0 - 67.0 * es6 /3024.0);
My supposition is that this projection is being done based on a spherical
approximation for the earth, whereas I gather PROJ.4 actually does the
calculation based on an ellipsoidal model. Does people believe this to be
A few questions:
o Is the approach to computing "Ra" above a standard method? Should I get
equivelent results if force PROJ.4 to use a spherical earth model with the
same value as generated above?
o Is there an easy way of finding which GCTP and Geotrans projection suffer
from this approximation?
o Is there a reason why NIMA (and the USGS for GCTP) consider it acceptable
to use spherical approximations for some of these projections?
o How are those of us (like myself) trying to achieve data interoperability
between different systems supposed to handle these problems?
o Are there good venues (I mean online, not conferences) for discussion of
issues like this?
o Are GCTP and/or Geotrans like to change the way they handle this (and other
spherical projections)? Is there someone we can contact to encourage them?
I have directed replies to osrs-proj, as it seems like the best venue for
I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerda
and watch the world go round - Rush | Geospatial Programmer for Rent
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