On vendredi 25 août 2017 18:25:36 CEST Joaquim Luis wrote: > 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? >
Yes that's not obvious but internally SRS in GDAL are not modelled as a proj.4 string, but as WKT. So there's a importFromProj4() and exportFromProj4(), and possible loss can happen when some concepts cannot be matched exactly. > Found these two that not even with the +wktext agree Perhaps you don't use the same proj.4 version ? > > > 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 I get the same result as GDAL (with proj 4.9.2) $ echo 4.897 52.371 | proj +proj=aeqd +ellps=WGS84 +units=m 384537.46 5809944.81 > > > > 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 $ echo 4.897 52.371 | proj +proj=robin +ellps=WGS84 +units=m 394576.53 5570940.84 > > > Joaquim, > > > >> I'm implementing Proj.4 in GMT via GDAL and now, in the the testing > >> stage, > >> > >> > >> > >> I'm using data from > >> > >> > >> > >> > >> > >> > >> > >> https://github.com/Beman/boost-trunk-git-svn/blob/master/libs/geometry/te > >> st_ > >> > >> > >> > >> extensions/gis/projections/projections.cpp#L121 > >> > >> > >> > >> > >> > >> > >> > >> but to my surprise lots of projections are not implemented in GDAL. For > >> > >> > >> > >> example > > > > GDAL SRS model is based on WKT, so only projections that have a mapping > > to WKT 1 (and for which a >developer cared enough to write the needed > > code !) are nominally supported > > > >> gdaltransform -s_srs EPSG:4326 -t_srs +proj=airy +ellps=WGS84 +units=m > > > > 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 > > > > > > > > > > > > > > $ gdalsrsinfo "+proj=airy +ellps=WGS84 +units=m +wktext" > > > > > > > > > > > > > > PROJ.4 : +proj=airy +ellps=WGS84 +units=m +wktext > > > > > > > > > > > > > > OGC WKT : > > > > > > > > PROJCS["unnamed", > > > > GEOGCS["WGS 84", > > > > DATUM["unknown", > >
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