[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):

 Hi,

 With the latest svn trunk I still see the failure to apply a datum shift
 as seen in this old screenshot:
 http://trac.osgeo.org/qgis/attachment/ticket/1079/qgis_fly_nodatum.png

 The problem is that QGIS fails to convert the DATUM string in the WKT
 shapefile.prj:

 {{{
 PROJCS["New Zealand Map Grid",
   GEOGCS["international",
     DATUM["New_Zealand_Geodetic_Datum_1949",
       SPHEROID["international",6378388,297]],
     PRIMEM["Greenwich",0],
     UNIT["degree",0.0174532925199433]],
   PROJECTION["New_Zealand_Map_Grid"],
   PARAMETER["latitude_of_origin",-41],
   PARAMETER["central_meridian",173],
   PARAMETER["false_easting",2510000],
   PARAMETER["false_northing",6023150],
   UNIT["meter",1]]
 }}}

 specifically the translation magic fails to add the +datum=nzgd49 term
 from: {{{ DATUM["New_Zealand_Geodetic_Datum_1949", }}}

 If I specifically pick `NZ Map Grid` from the CRS list it adds that term
 and the WGS84/LL and NZMG coastlines (more of less) correctly overlap, but
 I could not find a way to edit the proj4 terms manually in the layer
 projection settings. If the "*Generated CRS" is to be automagically
 converted into proj4 +terms, it would be nice if it were left in an
 editable state for those cases, either in the CRS picker GUI or in the
 layer properties GUI's General tab.   Maybe I am missing some easy trick
 there.

 So that gets rid of the 100m+ offset due to the wrong datum (ellipsoid).
 But a 2-4m shift remains due the the inability to apply +towgs84 or
 +nadgrids term. For +datum=nzgd49 there are 3 common datum transform
 options: 3-term +towgs84, 7-term +towgs84, and a +nadgrids NTv2 distortion
 grid file. One of these maps was reprojected from the other using the grid
 file.

 The +towgs84 datum transform terms are not saved in the shapefile's .prj
 WKT by GRASS export [IIRC you can add the +proj4 terms as a meta-data
 extra string in the WKT- that's a todo for grass's v.out.ogr module;
 somewhere Frank had an example of how that should look. perhaps on the
 proj4 FAQ or OGR's WKT parsing? I can't find it now...argh]



 OT:
 I'm finding it very difficult to test these as the map is taking about 2
 minutes to render after each adjustment (svn trunk on a brand new amd
 64bit quad-core + 8gig RAM). It's a large shapefile with the entire
 countries coastline as a set of large single-line polygons, but the .shp
 filesize is only 9MB so it's not that huge. Old versions of QGIS (0.8) had
 no problem with it. I believe this is happening even before on the fly
 reprojection is switched on.
 If this were GRASS I'd call it the classic "Florida Coastline" problem.


 I am happy to send anyone shp.zip's if it would help :)


 ramble,
 Hamish

-- 
Ticket URL: <http://trac.osgeo.org/qgis/ticket/1875#comment:34>
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