[gdal-dev] OGR GeoJSON transormation woes

Frank Broniewski hallo at frankbroniewski.com
Thu May 7 02:52:31 PDT 2020

Hi Even,

thank you very much for the hint. Although after researching on it I 
fell into a rabbit hole and just barely got out, but with a decent 
headache ;-)
Why do these coordinate systems and their axes have to be so 
complicated, they make my head hurt ... :-)

For future readers on the topic of coordinate systems, axes and the 
order of things, there is a nice Wiki article on the OSGeo website, 
aptly named Axis Order Confusion [1] where you can read about the stuff, 
follow the links and get a headache too :-)

Onwards to the solution of my problem, which I want to share of course. 
Even's hint suggested to look at the axis ordering of the LUREF CRS, 
which is "northing (X)" and then "easting (Y)". Reading [2] more 
carefully might have given me already a hint about what to do, since the 
page says:

"Starting with GDAL 3.0, the axis order mandated by the authority 
defining a CRS is by default honoured by the OGRCoordinateTransformation 
class, and always exported in WKT1"

which should have lead me to the conclusion that EPSG:2169 also has the 
axes swapped. And if my memory would have served me better, I once knew 
this fact as well.

Well, apparently all I needed to to was to tell my target 
SpatialReference to honour OAMS_TRADITIONAL_GIS_ORDER as well:

is all that was needed ...

But after reading all this stuff, I wonder if it would not have been 
better/more right to "correct" the GeoJSON coordinate order from Lon/Lat 
to Lat/Lon so that it matches the definition of EPSG:4326? I will have 
to think about how to deal with geo data and the treatment of CRS in 
future projects. Simply setting SetAxisMappingStrategy where it may fit 
does not seem to be a very thought-out and future proof process in the end.

is the API documentation for Python up to date? The SpatialReference 
docs [3] are missing the SetAxisMappingStrategy method

[1] https://wiki.osgeo.org/wiki/Axis_Order_Confusion
[2] https://gdal.org/tutorials/osr_api_tut.html
[3] https://gdal.org/python/osgeo.osr.SpatialReference-class.html

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