[GRASSLIST:3059] Re: Understanding projections under grass.

[Eric G. Miller]

It's good to understand coordinate systems when working with GIS.  Basically
there are two types projected or geodetic/geographic (discounting unreferenced
coordinate systems).  A geodetic/geographic system is specified via a datum
or ellipsoid and prime meridian (it's really erroneous for GRASS to ask for
both, since a datum defines the ellipsoid, meridian, and some other reference
info).  A projected coordinate system is based on a geodetic/geographic
reference system.  The projection type specifies the method, but it is not
a coordinate system until all the the parameters have been specified.  Something
like UTM is just a special case of the more general transverse mercator
projection.  But is does not become a coordinate system until the 
geodetic/geographic reference and the UTM zone are specified.

In your case, if you just messed up in specifying the datum/ellipsoid,
you can rerun g.setproj.  Note: If you've projected data via an incorrect
datum/ellipsoid, it can have significant error.  For rectified imagery
it might not be so severe, provided you haven't also projected it.  I'm
thinking GRASS doesn't really look at datum/ellipsoid parameters when
doing rectification (only control points).  So, your probably safe.

-- 
Eric G. Miller <egm2 at jps.net>