Thanks Rich and Dylan I downloaded the pdf of document #1395. At the moment I am leaning toward Lambert Conic Conformal (1SP) since it seems to use Lat/Long of Natural Origin, in case I need to use a GPS. If I am reading you right Latitude and longitude don't even come into the equation, just the projection.
I've been looking at the website http://www.dmap.co.uk/utmworld.htm. I was mistaken it was 18 and 19T that NH falls in. However, it appears to be just the western most sliver. However, if I don't have to figure out the conversion, so much the better. Kurt On May 14, 2010, at 12:00 PM, [email protected] wrote: > > Message: 2 > Date: Fri, 14 May 2010 08:10:20 -0700 > From: Dylan Beaudette <[email protected]> > Subject: Re: [GRASS-user] Latitude/Longitude vs UTM > To: Rich Shepard <[email protected]> > Cc: GRASS user list <[email protected]> > Message-ID: > <[email protected]> > Content-Type: text/plain; charset=ISO-8859-1 > > On Fri, May 14, 2010 at 6:22 AM, Rich Shepard <[email protected]> > 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 >> _______________________________________________ >> grass-user mailing list >> [email protected] >> http://lists.osgeo.org/mailman/listinfo/grass-user
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