[GRASS5] projection-transformation

[Roger S. Miller]

On Thu, 11 Jan 2001, Markus Neteler wrote:


> Let me say: I am slightly confused.
> Perhaps the problem is that the map's projection is Lambert, but GRASS
> et al. treat is as Geographic (lat/long)? But, then, how to get in
> a map projection in Lambert if only lat/long coordinates are printed on
> that map?

I wouldn't expect you to get much accuracy from trying to digitize
latitude and longitude off a map in lambert coordinates, but as long as
the lambert coordinates were appropriate for the map area it doesn't seem
like the errors should be large.

I can't help much with the digitizing work, sorry.  If you know the actual
latitude and longitude of several of the digitized points then you might
be able to use those in v.transform to adjust the entire file to more
nearly correct coordinates.

More rigorously, it seems like what you need are the lambert coordinates
of the map's visible latitude-longitude tics, or other points on the map
that you can use to register the map.  You might be able to do something
like:  1) set up a mapset using the lambert specification for the map, 2)
build a sites file in your latlong mapset that contains the latitude and
longitude tics marked on the map; 3) use s.proj to convert the tic
locations from latlong to lambert. Use the lambert coordinates of the
latitiude-longitude tics to register the map and then redigitize in
lambert coordinates.


Roger Miller
Lee Wilson and Associates


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