[gdal-dev] Which Proj.4 transforms are available in GDAL?

[Joaquim Luis]

OK, understood thanks. But got confused too. Why would a GDAL port of a  
proj.4 string do a different thing than the coded in the proj.4 string?

Found these two that not even with the +wktext agree


gdaltransform -s_srs EPSG:4326 -t_srs "+proj=aeqd +ellps=WGS84 +units=m  
+wktext"
4.897 52.371
384537.462362467 5809944.80754693 0

echo 4.897 52.371 | proj +proj=aeqd +ellps=WGS84 +units=m
384923.72       5809986.5



gdaltransform -s_srs EPSG:4326 -t_srs "+proj=robin +ellps=WGS84 +units=m  
+wktext"
4.897 52.371
394576.527707384 5570940.8420985 0

echo 4.897 52.371 | proj +proj=robin +ellps=WGS84 +units=m
394576.51       5571243.64




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> Joaquim,
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>> I'm implementing Proj.4 in GMT via GDAL and now, in the the testing  
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>> I'm using data from
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>> https://github.com/Beman/boost-trunk-git-svn/blob/master/libs/geometry/test_
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>> extensions/gis/projections/projections.cpp#L121
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>> but to my surprise lots of projections are not implemented in GDAL. For
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>> example
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> GDAL SRS model is based on WKT, so only projections that have a mapping  
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> code !) are nominally supported
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>> gdaltransform -s_srs EPSG:4326 -t_srs +proj=airy +ellps=WGS84 +units=m
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> For unsupported projections, add +wktext. A dummy WKT representation is  
> then built that captures >the full proj.4 string in a EXTENSION node.  
> Can also serve for supported projections if you want to >overrite the  
> WKT -> proj.4 string conversion done by GDAL
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> $ gdalsrsinfo "+proj=airy +ellps=WGS84 +units=m +wktext"
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> PROJ.4 : +proj=airy +ellps=WGS84 +units=m +wktext
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> OGC WKT :
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> PROJCS["unnamed",
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>    GEOGCS["WGS 84",
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>        DATUM["unknown",
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>            SPHEROID["WGS84",6378137,298.257223563]],
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>        PRIMEM["Greenwich",0],
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>        UNIT["degree",0.0174532925199433]],
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>    PROJECTION["custom_proj4"],
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>    UNIT["Meter",1],
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>    EXTENSION["PROJ4","+proj=airy +ellps=WGS84 +units=m +wktext"]]
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> $ gdaltransform -s_srs EPSG:4326 -t_srs "+proj=airy +ellps=WGS84  
> +units=m +wktext"
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>> ERROR 1: Translating source or target SRS failed:
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>> +proj=airy
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>> Or
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>> gdaltransform -s_srs EPSG:4326 -t_srs +proj=bonne
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>> ERROR 6: Failed to initialize PROJ.4 with `+proj=bonne +lon_0=0 +lat_1=0
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>> +x_0=0 +y_0=0 +ellps=WGS84 +units=m +no_defs'.
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> I thing the issue is that lat_1=0 is invalid for bonne. Any non zero  
> value is OK
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>> Is there a place where I can find the list of available proj.4  
>> transforms?
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> You mean projections supported by GDAL (as far as I know, most if not  
> all projections supported by >GDAL translate then to proj.4) ?
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> Look at the various OSRSetXXXXX() methods in
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> http://gdal.org/ogr__srs__api_8h.html
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> Or starting there
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> https://github.com/OSGeo/gdal/blob/trunk/gdal/ogr/ogr_srs_proj4.cpp#L616
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>> And also found some that do no agree between GDAL and proj. For example
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>> echo 4.897 52.371 | proj +proj=vandg +ellps=WGS84 +units=m
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>> 489005.93       6431581.02
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>> gdaltransform -s_srs EPSG:4326 -t_srs "+proj=vandg +ellps=WGS84  
>> +units=m"
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>> 4.897 52.371
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>> 488459.293282675 6424391.46347316 0
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> If you add --debug on, you'll see that GDAL adds a +R_A parameter. I  
> haven't dug up into history to >know the reason for that.
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> If you add +wktext, the original string will be passed unmodified to  
> proj.4
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> Even
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> --
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> Spatialys - Geospatial professional services
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> http://www.spatialys.com