<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Thanks Rich and Dylan<div><br></div><div>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.</div><div><br></div><div>I've been looking at the website <a href="http://www.dmap.co.uk/utmworld.htm">http://www.dmap.co.uk/utmworld.htm</a>. 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.</div><div><br></div><div>Kurt<br><div><div>On May 14, 2010, at 12:00 PM, <a href="mailto:grass-user-request@lists.osgeo.org">grass-user-request@lists.osgeo.org</a> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><br>Message: 2<br>Date: Fri, 14 May 2010 08:10:20 -0700<br>From: Dylan Beaudette <<a href="mailto:dylan.beaudette@gmail.com">dylan.beaudette@gmail.com</a>><br>Subject: Re: [GRASS-user] Latitude/Longitude vs UTM<br>To: Rich Shepard <<a href="mailto:rshepard@appl-ecosys.com">rshepard@appl-ecosys.com</a>><br>Cc: GRASS user list <<a href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a>><br>Message-ID:<br><span class="Apple-tab-span" style="white-space: pre; "> </span><<a href="mailto:AANLkTimnbx-ED-oCXMdGTm_Pw5ngzM48CbabS98twsF4@mail.gmail.com">AANLkTimnbx-ED-oCXMdGTm_Pw5ngzM48CbabS98twsF4@mail.gmail.com</a>><br>Content-Type: text/plain; charset=ISO-8859-1<br><br>On Fri, May 14, 2010 at 6:22 AM, Rich Shepard <<a href="mailto:rshepard@appl-ecosys.com">rshepard@appl-ecosys.com</a>> wrote:<br><blockquote type="cite">On Thu, 13 May 2010, Kurt Springs wrote:<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><blockquote type="cite">This was interesting in that it told me that r.topidx could not be run<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">with latitude and longitude and I had to convert to UTM. I was wondering<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">if this is the answer to the problem and I just had to convert to UTM.<br></blockquote></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Kurt,<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"> Lat/Long represents geographic coordinates, not a projection of location<br></blockquote><blockquote type="cite">on a mathematial model of the earth. UTM is the Universal Transverse<br></blockquote><blockquote type="cite">Mercador projection that we see on most printed (or computer displayed) maps<br></blockquote><blockquote type="cite">of the earth. There is documentation within the GRASS Web site that provides<br></blockquote><blockquote type="cite">a good explanation of the differences. GRASS modules work on geographic<br></blockquote><blockquote type="cite">projections, not just coordinates.<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"> There is a USGS technical report from the mid-1980s that's the standard on<br></blockquote><blockquote type="cite">projections. While it is becoming more rare to locatate, see if you can find<br></blockquote><blockquote type="cite">a copy.<br></blockquote><br>I think that Rich is referring to this USGS document, #1395<br><br><a href="http://pubs.er.usgs.gov/usgspubs/pp/pp1395">http://pubs.er.usgs.gov/usgspubs/pp/pp1395</a><br><br>Definitely worth the price if you want to become an expert in map projections.<br><br><br><blockquote type="cite"><blockquote type="cite">One other question. New Hampshire appears to fall within two UTM zones<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">(19T and 20T) Is there a way for a maps set to contain two UTM zones?<br></blockquote></blockquote><br>Yes. Don't use UTM. In this case use a regional projection that suits<br>your needs:<br><br>1) navigation --> use a conformal projection<br>2) area statistics --> use an equal-area projection<br>... etc ...<br><br>Variations on the Albers or Lambert (conformal) conic projections work<br>quite well for large regions that are wider than tall, but for such as<br>small state should be just fine. We use an Albers equal-area<br>projection to house soil survey data for the lower 48 states.<br><br><blockquote type="cite"> Interesting. NH is a tall, narrow state so one would assume it would be<br></blockquote><blockquote type="cite">within a single zone. Regardless, yes there is a way to reproject locations<br></blockquote><blockquote type="cite">in one zone on the other, but it's non trivial and I've not done it.<br></blockquote><br>I wouldn't recommend it. The desirable properties of the UTM system<br>(i.e. the fairly good compromise between distortion, preservation of<br>angles, and preservation of area) only occur within a zone's<br>boundaries. The farther you move from the central meridian of the UTM<br>zone, the more distortion you will encounter-- therefore 'projecting'<br>UTM z10 data into UTM z11 is technically possible, but not a great<br>idea.<br><br><blockquote type="cite"> Oregon is primarily in Zone 10, but the eastern edge (I don't recall the<br></blockquote><blockquote type="cite">distance within the state) is in Zone 11. The available DEM and hydrologic<br></blockquote><blockquote type="cite">data were reprojected from 11 to 10 by the supplying agency.<br></blockquote><br>Hmm...<br><br>Dylan<br><br><blockquote type="cite">Rich<br></blockquote><blockquote type="cite">_______________________________________________<br></blockquote><blockquote type="cite">grass-user mailing list<br></blockquote><blockquote type="cite"><a href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a><br></blockquote><blockquote type="cite"><a href="http://lists.osgeo.org/mailman/listinfo/grass-user">http://lists.osgeo.org/mailman/listinfo/grass-user</a></blockquote></blockquote></div><br></div></body></html>