[GRASS5] Reading Projections

GRASS GRASS developer
1

Folks,

As part of the OSSIM/GRASS bridge project, I would like to implement good
access to the coordinate system of GRASS files. I have looked through the
GRASS 5 Programmers Manual, but I am still a bit unclear on how I am supposed
to read the full projection information for a raster (cell). The
G_get_cellhd() function allows me to get the projection number, and zone, but
doesn't appear to have support for additional parameters of complex projections
(like Transverse Mercator's central meridian for instance). The
G_get_projinfo() call (used by r.proj) appears to get a full set of parameters,
but does so by reading the locations PROJ_INFO and PROJ_UNITS files. It seems
to me that the location's default projection might be different than the
projection of a particular raster in that location ... is that right?

Finally, I am interested in getting GRASS data samples of as many supported
projections as is practical. I am also interested in getting datasets
containing a few datum definitions done using the new datum methods in GRASS5.

Best regards,

---------------------------------------+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, warmerda@home.com
light and sound - activate the windows | http://members.home.com/warmerda
and watch the world go round - Rush | Geospatial Programmer for Rent

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2

On Thu, 21 Sep 2000, Frank Warmerdam wrote:

It seems to me that the location's default projection might be
different than the projection of a particular raster in that location
... is that right?

No. Every raster or vector map in a particular location share the same
projection (described in PROJ_INFO). Maps, like regions within a grass
location, can have different borders and even different resolutions
(descrbed in WIND or DEFAULT_WIND) but they are all based on same
projection.

Whether this restriction will be necessary in the future I am not sure.
The speed of today's hardware and software may already allow 'dynamic'
projections so each region/map is projected on the fly. We could then have
a DEFAULT_PROJ_INFO for the location, and a PROJ_INFO file for each region
or map within that location, just like we have DEFAULT_WIND and WIND. This
would make it unnecessary to define several locations for the same
geographic area, which only differ in projection and save a lot of disk
space. Maybe this is something GRASS6 or later ...

Morten Hulden

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