[QGIS-trac] Re: [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:  closed       
        Priority:  critical: causes crash or data corruption  |     Milestone:  Version 1.2.0
       Component:  Projection Support                         |       Version:  HEAD         
      Resolution:  fixed                                      |      Keywords:               
Platform_version:                                             |      Platform:  All          
        Must_fix:  No                                         |   Status_info:  0            
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Comment (by hamish):

 > I still can't find Frank's post about storing +proj terms in
 > a WKT extra field. Frustrating.

 Ok, I found him on IRC. the WKT node for the +proj terms is:
 {{{
    EXTENSION["PROJ4","+proj=latlong +datum=WGS84 +wktext"]]
 }}}

 but a) that isn't ESRI compatible [.prj/map will be rejected] and b) OGR
 export rips out any WKT terms which are not ESRI compatible. So no luck
 there.

 There is also a WKT node called `TOWGS84[]` in the CT spec (see
 wktprobles.html above) but that is also not ESRI compatible so no luck
 there either.

 I am not sure, but I feel that ESRI at least supports
 `AUTHORITY["EPSG:4326"]` style, FWTW.

 Another thing Frank mentioned is that GeoTiff has no mechanism to store
 TOWGS84 parameters.

 My reading of all this is that datum transform params will not transfer
 well from WKT. My perspective is wondering how to improve GRASS's
 v.out.ogr and r.out.gdal modules to provide as many hints as possible for
 QGIS et al. to easily pick up.  The ''only'' idea I know of currently is
 to distribute a .pj4 file containing +proj terms alongside the .prj WKT
 file, as +proj is the only format which is expressive enough to preserve
 CRS fidelity.


 Hamish

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Ticket URL: <http://trac.osgeo.org/qgis/ticket/1875#comment:37>
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