[GRASS-user] Latitude/Longitude vs UTM

[Dylan Beaudette]

On Fri, May 14, 2010 at 6:22 AM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> 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.

I think that Rich is referring to this USGS document, #1395

http://pubs.er.usgs.gov/usgspubs/pp/pp1395

Definitely worth the price if you want to become an expert in map projections.


>> 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?

Yes. Don't use UTM. In this case use a regional projection that suits
your needs:

1) navigation --> use a conformal projection
2) area statistics --> use an equal-area projection
... etc ...

Variations on the Albers or Lambert (conformal) conic projections work
quite well for large regions that are wider than tall, but for such as
small state should be just fine. We use an Albers equal-area
projection to house soil survey data for the lower 48 states.

>  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.

I wouldn't recommend it. The desirable properties of the UTM system
(i.e. the fairly good compromise between distortion, preservation of
angles, and preservation of area) only occur within a zone's
boundaries. The farther you move from the central meridian of the UTM
zone, the more distortion you will encounter-- therefore 'projecting'
UTM z10 data into UTM z11 is technically possible, but not a great
idea.

>  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.

Hmm...

Dylan

> Rich
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