[gdal-dev] Coordinate transformation of geometries between CRSs with different axis ordering (C API)

[Even Rouault]

Le 01/04/2026 à 16:04, Florian Katerndahl via gdal-dev a écrit :
> Dear all,
>
> I'm currently working on a small program which allows users to specify 
> vector inputs in different coordinates systems and that potentially 
> "normalizes" them to EPSG:4326 internally to process them further. 
> Maybe I'm too inexperienced and there's an easy solution but I got 
> hung up on the coordinate transformation when the input CRS has a 
> different axis ordering compared to the output CRS using GDAL's/OGR's 
> C API.
>
> For example, transforming between EPSG:5556 and EPSG:4326, GDAL 
> outputs a warning (forwarded from proj?) that the output's latitude is 
> invalid. Setting the axis ordering of the target CRS to 
> `OAMS_TRADITIONAL_GIS_ORDER` fixes this and results in correctly 
> located polygons. My question is now: Is it sufficient/correct to 
> compare the axis ordering of the input and output coordinate systems 
> and set above mentioned flag (or a custom mapping when neither "x"/"y" 
> are the first two axis) in case they differ?

The axis ordering of the input CRS vs the one of the output CRS is 
complelely unrelated w.r.t your decision about setting or not 
OAMS_TRADITIONAL_GIS_ORDER. Setting it or not for the input totally 
depends on your decision how you want to specify the coordinates.


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