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

[Javier Jimenez Shaw]

EPSG:4326 is lat-lon, EPSG:5556 is easting-northing-up.
I don't see where is your problem.
It would be easier if you provide some numbers and tell exactly what you
expect (with numbers).

On Wed, 1 Apr 2026 at 17:14, Florian Katerndahl via gdal-dev <
gdal-dev at lists.osgeo.org> wrote:

> Hi Even,
>
> thanks for your reply.
>
> I don't pass the coordinates directly via `OCTTransform` but use
> `OGR_GeomTransformer_Transform` to transform polygon geometries. Based
> on the fact than when fetching the points that make up the input
> polygon, the x and y coordinates match the axis of the input CRS, my
> assumption was that I don't need to change the ordering for the input
> CRS. Looking at the points in the output geometry, the coordinates are
> flipped when not setting OAMS_TRADITIONAL_GIS_ORDER. This was the reason
> why I assumed I need to set the axis ordering depending on the CRSs I
> translate between.
>
> But if I understand your reply correctly, I should also not need to
> touch the axis ordering for the output CRS as GDAL takes care of mapping
> the data to CRS axis for the output geometry. I suppose my understanding
> is not sufficient enough (yet) to understand the underlying issue then
> and I will need to do some further research on my side.
>
>
> Kind regards,
>
> Florian
>
>
> On 4/1/26 16:26, Even Rouault wrote:
> >
> > 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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