[GRASSLIST:2569] Re: Multiple UTM zones

[Rich Shepard]

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).

Glynn,

  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
surface.

> 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,

Rich

Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
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