[Qgis-user] Proyection cuestion?

[Gerardo Jimenez]

Hi
First Qgis use a projection engine called proj4 which uses EPSG codes.  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=) 
+proj=utm +zone=14 +datum=NAD27 +units=m +no_defs
This is the epsg code modified to transform between nad27 and wgs 84 for Mexico only. 
+proj=utm +zone=14 +ellps=clrk66 +towgs84=-12,130,190,0,0,0,0 +units=m +no_defs
As you can notice it does contain a parameter for the datum shift valid only for Mexico ( +towgs84=-12,130,190,0,0,0,0) .  This parameters came from various sources, one of them:
National Imagery and Mapping Agency,2000, Departament of Defense World Geodetic System 1984: its definition and relationships with local geodetic systems.
Arc Map uses the same parameters . Also check this
http://earth-info.nga.mil/GandG/coordsys/onlinedatum/CountryNorthAmericaTable.html#NASL
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 " +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.
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.
For raster layers, you have to go to raster menu/ projection/warp(reproject).
Check this, I cover some basics in Qgis
http://www.youtube.com/user/Arqueovader
Hope it helps
Gerardo Jiménez Delgado
Instituto de Investigaciones Antropológicas
Universidad Nacional Autónoma de México
Ciudad Universitaria s/n
Coyoacan 04510
Mexico City
Mexico

56 22 95 16

From: jm_bernales at hotmail.com
To: ingguerrero7 at gmail.com; qgis-user at lists.osgeo.org
Date: Wed, 17 Jul 2013 15:37:59 -0400
Subject: Re: [Qgis-user] Proyection cuestion?




Hi Roberto,
I'm Juan from México, if you want you can ask in spanish
You can define On The Fly Transformation, in the progect properties

SALUDOS

JUAN M. BERNALES

 
Date: Wed, 17 Jul 2013 10:55:49 -0500
From: ingguerrero7 at gmail.com
To: qgis-user at lists.osgeo.org
Subject: [Qgis-user] Proyection cuestion?

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.

 Greetings
Roberto Guerrero Renteria



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