[Proj] Lambert tangent form to Lambert secant form

Clifford J Mugnier cjmce at lsu.edu
Sun Jan 13 17:56:43 EST 2008

The Lambert Conformal Conic with one standard parallel defined with  a latitude of origin and a scale factor at origin less than one is a secant Lambert.  There are very few truly tangent Lambert Conformal Conic zones extant in the entire world.  The difference is what you have a problem with is likely just some algebra.  See Chapter 3 of the current "Manual of Photogrammetry" published by the American Society for Photogrammetry and Remote Sensing for some algebraic clues to your conundrum.
C. Mugnier
Louisiana State University


From: proj-bounces at lists.maptools.org on behalf of RICHARD Didier
Sent: Sun 13-Jan-08 12:18
To: proj at lists.maptools.org; gdal-dev at lists.osgeo.org
Subject: [Proj] Lambert tangent form to Lambert secant form

Hi all,

I am facing a bug in the MapInfo driver of GDAL that might need some
modifications of PROJ.4. The bug is two fold :

- MapInfo driver only supports 1SP LCC when building the CoordSys string.
Before, I implement an algorithm that modify LCC 1SP parameters to LCC 2SP
parameters. I would like to know whether someone on the list has already
implemented such a code. Another point is how to wrap that in GDAL (I
address this question to those who are on both list);

- When I use a SRS definition with both nadgrids and towgs84 parameters in
a PROJ.4 string, the later is not taken into account. Therefore, a WKT
string of such a SRS lacks the TOWGS84 block. As a consequence, the
MapInfo driver is not able to code the datum correctly (104 is written
which is the WGS84 datum). I would like to suggest to modify
pj_datum_set.c to keep storing towgs84 values even when nadgrids have been
encountered. The idea is to keep the datum_type has PJD_GRIDSHIFT in order
to keep precise coordinates transformation with grid and to parse the
towgs84 (from line 113). I don't know if there are known or already
debated issues on that, but I would like to know whether this idea is
correct or not.

Any comments/suggestions ?


RICHARD Didier - Chef du pôle technique du Géoportail
2/4, avenue Pasteur - 94165 Saint Mandé Cedex
Tél : +33 (0) 1 43 98 83 23
Proj mailing list
Proj at lists.maptools.org

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