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<body class='hmmessage'><div dir='ltr'>Hi<div><br></div><div>First <span style="font-size: 12pt;">Qgis use a projection engine called proj4 which uses EPSG codes. </span><span style="font-size: 12pt;"> The parameters for datum change from nad27 to wgs 84 for Mexico are a little bit wrong in qgis. This is an example of the UTM zone 14 with a nad27 datum from qgis, it does not have a a parameter to define the datum shift (+towgs84=) </span></div><div><span style="font-size: 12pt;"><br></span></div><div>+proj=utm +zone=14 +datum=NAD27 +units=m +no_defs</div><div><br></div><div>This is the epsg code modified to transform between nad27 and wgs 84 for<b> Mexico only</b>. </div><div><br></div><div>+proj=utm +zone=14 +ellps=clrk66 +towgs84=-12,130,190,0,0,0,0 +units=m +no_defs</div><div><br></div><div><div>As you can notice it does contain a parameter for the datum shift valid only for Mexico (<span style="font-size: 12pt;"> </span><span style="font-size: 12pt;">+towgs84=-12,130,190,0,0,0,0)</span><span style="font-size: 12pt;"> . This parameters came from various sources, one of them:</span></div><div><span style="font-size: 12pt;"><br></span></div><div><span style="font-size: 12pt;">National Imagery and Mapping Agency,</span></div><div><span style="font-size: 12pt;">2000, Departament of Defense World Geodetic System 1984: its definition and relationships with local geodetic systems.</span></div><div><span style="font-size: 12pt;"><br></span></div><div>Arc Map uses the same parameters . Also check this</div><div><br></div><div><a href="http://earth-info.nga.mil/GandG/coordsys/onlinedatum/CountryNorthAmericaTable.html#NASL" target="_blank">http://earth-info.nga.mil/GandG/coordsys/onlinedatum/CountryNorthAmericaTable.html#NASL</a></div><div><br></div><div><span style="font-size: 12pt;">Having say that, The first thing you have to do is to define the code for the coordinate system you want to transform, take the utm defined above as an example. Just make sur that the code includes the "</span><span style="font-size: 12pt;"> </span><span style="font-size: 12pt;">+towgs84=-12,130,190,0,0,0,0" parameter. Then you have to include it in Quantum as a custom coordinate reference system. To do so, go to settings/ custom crs. All CRS you create will appear as user defined coordinate systems in the coordinate systems selector.</span></div><div><span style="font-size: 12pt;"><br></span></div><div>For vector layer you can make the transformations (reprojection) by selecting the layer, right click and choose save as. You have to define the kind os file (shp, for instance) a name and location and also the new coordinate system.</div><div><br></div><div>For raster layers, you have to go to raster menu/ projection/warp(reproject).</div><div><br></div><div>Check this, I cover some basics in Qgis</div><div><br></div><div><a href="http://www.youtube.com/user/Arqueovader" target="_blank">http://www.youtube.com/user/Arqueovader</a></div><div><br></div><div>Hope it helps</div><br>Gerardo Jiménez Delgado<br>Instituto de Investigaciones Antropológicas<br>Universidad Nacional Autónoma de México<br>Ciudad Universitaria s/n<br>Coyoacan 04510
Mexico City<br>Mexico
56 22 95 16<br><br><div><hr id="stopSpelling">From: jm_bernales@hotmail.com<br>To: ingguerrero7@gmail.com; qgis-user@lists.osgeo.org<br>Date: Wed, 17 Jul 2013 15:37:59 -0400<br>Subject: Re: [Qgis-user] Proyection cuestion?<br><br>
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<div dir="ltr">Hi Roberto,<br>I'm Juan from México, if you want you can ask in spanish<br>You can define On The Fly Transformation, in the progect properties<br><br><em><font face="Comic Sans MS">SALUDOS</font></em><br>
<em><font face="Comic Sans MS">JUAN M. BERNALES</font></em><br><br> <br><div><hr id="ecxstopSpelling">Date: Wed, 17 Jul 2013 10:55:49 -0500<br>From: ingguerrero7@gmail.com<br>To: qgis-user@lists.osgeo.org<br>Subject: [Qgis-user] Proyection cuestion?<br><br><div dir="ltr">Hi, good day. I am from México, and I want to know, how can I proyect from UTM Nad27 Mexico to WGS84, and viceverse? in vectorial and raster.<br><div><br></div><div> Greetings</div><div><br></div><div>Roberto Guerrero Renteria</div>
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