[GRASSLIST:2569] Re: Multiple UTM zones

Rich Shepard rshepard at appl-ecosys.com
Mon Oct 8 14:48:34 EDT 2001

On Mon, 8 Oct 2001, Glynn Clements wrote:

> UTM isn't really "a projection"; it's a family of 60 projections. For
> the most part, GRASS won't care if you don't use the "right" zone
> (beyond the fact that the distortion increases as you get further from
> the tangent meridian).


  I won't argue semantics with you, but I always understood that the
Universal Transverse Mercator was a projection of the 3D Earth onto a flat

> Actually, GRASS doesn't really deal with projections. Most programs
> just operate upon abstract 2D coordinates, treating the region as if
> it were flat.

  Excellent point. But, aren't distances and directions dependent upon the
projection used? Otherwise, why do all GIS software provide for
transformations among projections and why do different layers/themes/maps
have to be in the same projection/coordinate system so that different
objects are in proper relative postitions to each other?

> To get down to specifics: if you have data from several zones, and
> want to import it into a single location, you will probably need to
> either:

  My concern -- not based on a current project -- is when a project site
spans the boundary of two UTM zones.

> a) convert the coordinates using m.proj, or

  As long as this is an acceptable activity I'll use it.

Many thanks,


Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
            2404 SW 22nd Street | Troutdale, OR 97060-1247 | U.S.A.
 + 1 503-667-4517 (voice) | + 1 503-667-8863 (fax) | rshepard at appl-ecosys.com

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