[GRASS-dev] reading georeferencing tables for GRASS

[Michael Barton]

Paul and Glynn,

I'm ready to move ahead and try to create PROJ.4 strings in the wxgrass
location wizard, but am confused about the parameters tables in
$GISBASE/etc.

It looks kind of like proj-params.table and maybe proj-units.table are the
files used by g.setproj and that the others (datum.table,
datumtranform.table, ellipse.table, and projections) are there to help
create a PROJ.4 string for GRASS. Is this correct?

If this is the case, I'm not quite sure how to use these.

The datumtransforms.table seems the most straightforward--just search for
the line with a projection that matches the location projection and read of
the datum transformation parameters ("towgs84=24,-130,-92") as a single
PROJ.4 string to add on to the PROJ.4 string of the location projection.

I do use projections? That is, choose the projection code that is
appropriate for the description that I want.

the datum.table and ellipse.table are more confusing. The datum table seems
to include ellipsoids, but only a subset and does not include the ellipsoid
parameters. I can look up the ellipsoid in the ellipse.table file that
matches the ellipsoid in the datum.table file and grab the ellipsoid
parameters. But what about all the ellipsoids that are not in the
datum.table file? Can they be used?

It seems that, hypothetically, you could use a different ellipsoid with a
datum besides the one associated with it in the datum.table file. But I
don't know why those particular ellipsoids are in the datum.table file. Are
these simply the most common for that datum or do I *need* to use the
ellipsoid associated with that datum?

I hope that you can help me get it clear how these 2 tables are related so
that I can begin to parse them into a PROJ.4 string.

Thanks
Michael
__________________________________________
Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton


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