[GRASSLIST:3059] Re: Understanding projections under grass.
Eric G. Miller
egm2 at jps.net
Thu Jan 31 20:56:23 EST 2002
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>
More information about the grass-user