[QGIS-trac] [Quantum GIS] #1875: A few problems with crs definitions with TOWGS parameters

[Quantum GIS]

#1875: A few problems with crs definitions with TOWGS parameters
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   Reporter:  lutra                                      |              Owner:  homann       
       Type:  bug                                        |             Status:  new          
   Priority:  critical: causes crash or data corruption  |          Milestone:  Version 1.2.0
  Component:  Projection Support                         |            Version:  HEAD         
   Keywords:                                             |   Platform_version:  pcav, neteler
   Platform:  All                                        |           Must_fix:  No           
Status_info:  0                                          |  
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 Hi,

 I'm making some extensive test to try break down the problems described in
 the ticket #1079 and I may have found new issues (or not), one of them is
 new as changes made r11373.

 My start point is a layer with a crs definition that includes towgs
 parameters. You can find an example here

 https://trac.osgeo.org/qgis/attachment/ticket/1079/Gauss_Boaga.tar.gz

 a) after r11373, when you open such a layer the towgs parameters are
 stripped from the original crs string. This doesn't happened *before*
 r11373. In the specific case the layer is now recognized as having a
 "plain" 3003 crs, but it is not true, it misses the original towgs
 parameters

 [[BR]]


 b) if I try to save a layer as a new shapfile, qgis ask em to choose the
 crs. If a crs with towgs parameters is selected, in the final result the
 parameters do not appear. You can try doing the following:

 1) define a custom crs with the following string

 "+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl
 +towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68 +units=m +no_defs"

 and now try to save the layer in the above location as shapefile and try
 apply to it the crs just created. The final result will have a crs like
 this

 "+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl
 +units=m +no_defs "


 [[BR]]


 c) if I have two identical layers, with the only difference that one has
 towgs parameters in his crs, I can see the shift '''ONLY''' if the project
 is defined in a geographic crs (ex. wgs84). If the project is defined in a
 projected crs qgis seems to not reproject correctly the layer with towgs
 parameters.


 [[BR]]


 I noticed also that on a copy of qgis 1.2 compiled from source against the
 dev verison of gdal (1.7), when I open one of the layers of the qgis
 sample dataset, qgis do not make to recognize as 2964. If then I create a
 custom crs with the same string, it associates correctly with the custom
 crs.


 [[BR]]


 I noticed also that when creating custom crs's and a trailing space is
 left at the end of the string, qgis fails to associate the layer crs to
 it.

-- 
Ticket URL: <https://trac.osgeo.org/qgis/ticket/1875>
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