[GRASSLIST:2577] Re: Multiple UTM zones

[Glynn Clements]

Rich Shepard 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
> surface.

That depends upon the definition of "projection". Each UTM zone
defines a different (continuous) mapping between lat/lon values and
Northing/Easting values.

A UTM "coordinate" is a triple of zone/Northing/Easting; the last two
values are meaningless without the zone.

> > 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?

Yes. But, for the most part, the only information which GRASS actually
uses is a scale factor, so that it can convert units to physical
dimensions. Furthermore, the scaling is assumed to be the same in all
directions, and constant across the location.

The only code which really understands projections is libproj, and
only a few programs use that. The rest of GRASS generally acts as if
the location is a rectangle floating in space.

> Otherwise, why do all GIS software provide for
> transformations among projections

Because you may have multiple sets of data which use different
coordinate systems, but you need to use a single coordinate system
(although it usually doesn't matter which one).

> 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?

You've answered your own question here. Projections are only relevant
insofar as you need to convert coordinates between them.

To a large degree, you can lie about the projection and, so long as
you lie consistently, you'll still end up with the right answers.

-- 
Glynn Clements <glynn.clements at virgin.net>