[QGIS-trac] Re: [Quantum GIS] #1079: on the fly projections do not work

[Quantum GIS]

#1079: on the fly projections do not work
--------------------------------------------------------------+-------------
        Reporter:  pcav                                       |         Owner:  mhugent      
            Type:  defect                                     |        Status:  closed       
        Priority:  critical: causes crash or data corruption  |     Milestone:  Version 1.0.0
       Component:  Projection Support                         |       Version:  HEAD         
      Resolution:  fixed                                      |      Keywords:  srs.db datum 
Platform_version:                                             |      Platform:  All          
        Must_fix:  Yes                                        |   Status_info:  1            
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Comment (by msieczka):

 Replying to [comment:10 leolami]:

 > A little correlate problem is that if I load a layer from a Location of
 GRASS with
 > this parameters of projection:
 >
 > +proj=tmerc +lat_0=0 +lon_0=9 +k=0.999600 +x_0=1500000 +y_0=0
 > +ellps=intl +units=m
 +towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68
 >
 > when I look the projection associated to this layer I can see that QGIS
 doesn't know
 > this projection and that it associates to the layer the projection
 setted as default.
 >
 > But if I create a custom projection like this and I set it as the
 default projection
 > when I load the GRASS layer I see that QGIS associates to the layer the
 standard
 > projection:
 >
 > +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs
 >
 > but I remember that before QGIS readed the projection with parameters
 like the same
 > projection withot them and it associated to the layer the standard Gauss
 Boaga projection
 > (EPSG code=3003)

 The problem is that QGIS produces an exact, particular proj4 string for
 your layer, and tries to find a match for it in it's CRS database. In
 order to let it find it, you need to define a custom CRS using a prj4
 exactly the same QGIS qould be looking for.

 You can find the string QGIS is looking for in it's terminal output when
 it's loading the layer. In your case the very string is:

 {{{
 +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
 }}}

 The string *you* quoted is slightly different:

 {{{
 +proj=tmerc +lat_0=0 +lon_0=9 +k=0.999600 +x_0=1500000 +y_0=0 +ellps=intl
 +units=m +towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68
 }}}

 i.e.: different order of parameters, trailing zeros in 'k', missing
 'no_defs' - enough for QGIS to make it not recognize it.

 As long as QGIS depends on string-wise proj4 definitions matching, we need
 to keep an eye on such subtle differences.

 If you specify your custom CRS the way QGIS expects it, matching should
 work for you (it does for me).

 Maciek

-- 
Ticket URL: <https://trac.osgeo.org/qgis/ticket/1079#comment:15>
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