[GRASS-user] Latitude/Longitude vs UTM

[Rich Shepard]

On Thu, 13 May 2010, Kurt Springs wrote:

> This was interesting in that it told me that r.topidx could not be run
> with latitude and longitude and I had to convert to UTM. I was wondering
> if this is the answer to the problem and I just had to convert to UTM.

Kurt,

   Lat/Long represents geographic coordinates, not a projection of location
on a mathematial model of the earth. UTM is the Universal Transverse
Mercador projection that we see on most printed (or computer displayed) maps
of the earth. There is documentation within the GRASS Web site that provides
a good explanation of the differences. GRASS modules work on geographic
projections, not just coordinates.

   There is a USGS technical report from the mid-1980s that's the standard on
projections. While it is becoming more rare to locatate, see if you can find
a copy.

> One other question. New Hampshire appears to fall within two UTM zones
> (19T and 20T) Is there a way for a maps set to contain two UTM zones?

   Interesting. NH is a tall, narrow state so one would assume it would be
within a single zone. Regardless, yes there is a way to reproject locations
in one zone on the other, but it's non trivial and I've not done it.

   Oregon is primarily in Zone 10, but the eastern edge (I don't recall the
distance within the state) is in Zone 11. The available DEM and hydrologic
data were reprojected from 11 to 10 by the supplying agency.

Rich