[QGIS-trac] [Quantum GIS] #2913: a simple ogr2ogr interface will ease coordinate transformations with +towg84 or +nadgrids parameters

[Quantum GIS]

#2913: a simple ogr2ogr interface will ease coordinate transformations with
+towg84 or +nadgrids parameters
------------------------------------------------+---------------------------
   Reporter:  lutra                             |              Owner:  borysiasty   
       Type:  enhancement                       |             Status:  new          
   Priority:  major: does not work as expected  |          Milestone:  Version 1.6.0
  Component:  Python plugins and bindings       |            Version:  Trunk        
   Keywords:                                    |   Platform_version:               
   Platform:  All                               |           Must_fix:  No           
Status_info:  0                                 |  
------------------------------------------------+---------------------------
 QGIS is great because offer many ways/tools to save copies of vectors and
 change their projection along the process.

 But what about if you want to do the following (example)?


 {{{
 ogr2ogr -f "ESRI Shapefile" -s_srs "+proj=tmerc +lat_0=39.66666666666666
 +lon_0=-8.131906111111112 +k=1 +x_0=200000 +y_0=300000 +ellps=intl
 +units=m +towgs84=-282.1,-72.2,119.95,-1.53,0.14,-0.89,-4.5" -t_srs
 "+proj=tmerc +lat_0=39.6682583333474 +lon_0=-8.133108333465099 +k=1 +x_0=0
 +y_0=0 +ellps=GRS80 +towgs84=0,0,0 +units=m +no_defs" test.shp acid.shp
 }}}


 The problem is that there is no interface in QGIS that allows you to
 define manually the "-s srs"  or "-t srs" parameters (just for vectors,
 because the "gdal tools" tools do allow you to do so).

 You may think "so, what is the deal?".


 [[BR]]


 The other problem is that QGIS does not allow either to choose/define the
 "+towgs84" or "+nadgrids" parameters so to make things work you will
 basically have to:

 *) define a custom CRS, probably a clone of one already available but with
 the necessary "+towgs84" or "+nadgrids" parameters.

 *) save a copy of your vector giving to him the custom CRS with "+towgs84"
 or "+nadgrids" parameters

 *) save this vector in a new one with the -t srs coordinate system




 This is not really straightforward, but it works. So maybe until things
 will remain this way it would be useful to document this steps.

 [[BR]]


 The proposed ogr2ogr interface would be a very simple "plugin", and as a
 starting point you may want to have a look to the warp tool in the gdal
 tools plugin.

 Basically what is needed is a GUI that allows to:

 *) choose the input vector

 *) choose the output vector

 *) choose the input srs

 *) choose the output srs


 AND, if necessary

 *) choose/define +towgs84/+nadgrids parameters


 [[BR]]


 A few important notes:

 *) when the -t srs is a projected one, it must be defined with a
 "+towgs84=0,0,0" parameter, otherwise ogr2ogr will ignore the -s srs
 +towgs84/+nadgrids parameters.

 This is important because actually in QGIS you need ALSO to define a
 custom CRS (for the -t srs parameter) that holds the "+towgs84=0,0,0"
 string.

 *) NTV2 grids are pretty much more accurate than towgs84 parameters for
 large scale areas. Are binary files (*.gsb) and are freely available for
 many countries. To work with ogr2ogr they need to be copied to the
 appropriate system directory, under linux is


 {{{
 /usr/share/proj/
 }}}


 once there you can use them like this:



 {{{
 ogr2ogr -f "ESRI Shapefile" -s_srs "+proj=tmerc +lat_0=39.66666666666666
 +lon_0=-8.131906111111112 +k=1 +x_0=0 +y_0=0 +ellps=intl +units=m +no_defs
 +nadgrids=ptLX_e89.gsb +wktext" -t_srs "+init=epsg:4326" area_wgs84.shp
 area.shp
 }}}


 So for this interface I would also suggest a dropdown to show the already
 available .gsb files, and a way to let the users choose new .gsb files to
 be copied to the proper system location.

 [[BR]]

 Another important note:

 *) when you use the +nadgrids parameter you must also add the "+wktext"
 one, otherwise "+nadgrids" will be ignored/stripped.

-- 
Ticket URL: <https://trac.osgeo.org/qgis/ticket/2913>
Quantum GIS <http://qgis.org>
Quantum GIS is an Open Source GIS viewer/editor supporting OGR, PostGIS, and GRASS formats