[QGIS-trac] Re: [Quantum GIS] #2154: Not correctly creating (or reading) proj file for CRS for EPSG 2193

[Quantum GIS]

#2154: Not correctly creating (or reading) proj file for CRS  for EPSG 2193
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        Reporter:  mesajs                            |         Owner:  nobody       
            Type:  bug                               |        Status:  new          
        Priority:  major: does not work as expected  |     Milestone:  Version 1.4.0
       Component:  Projection Support                |       Version:  HEAD         
      Resolution:                                    |      Keywords:  CRS          
Platform_version:  Vista 64                          |      Platform:  Windows      
        Must_fix:  No                                |   Status_info:  0            
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Comment (by warmerdam):

 Tony,


 OK, that helps me.

 I have only limited familiarity with the mechanisms in QGIS for spatial
 reference system handling, though I'm quite familiar with PROJ.4 and the
 OGRSpatialReference class from GDAL (used to some extent).

 I see the current "trunk" PROJ.4 expansion of EPSG:2193 in GDAL is:

 +proj=tmerc +lat_0=0 +lon_0=173 +k=0.9996 +x_0=1600000 +y_0=10000000
 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

 This is similar to what QGIS used as a PROJ.4 string, except without the
 +towgs84 string indicating that the datum should be considered equivelent
 to WGS84.  I'm not sure why that is missing, perhaps something is older in
 your QGIS environment.

 This PROJ.4 string (in either form) provides no identifying information
 that would indicate the datum is NZGD2000 so there is no way to fill in
 the .prj file properly with a datum name.  Likewise, there is essentially
 not enough information left to relate the .prj file back to EPSG:2193.

 I *assume* QGIS is using the PROJ.4 string as a working representation of
 the spatial reference system, but this is a rather poor idea if it is
 desirable to relate things back to EPSG since PROJ.4 is quite inexpressive
 with regard to some details in the original EPSG definition or even the
 WKT definition.

 I think I'll leave this for someone more savvy with QGIS coordinate system
 support to comment, but at things stand there is little to be done to make
 this work without significant code restructuring.

-- 
Ticket URL: <https://trac.osgeo.org/qgis/ticket/2154#comment:3>
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