[gdal-dev] OGR Transformation Clarification

[Even Rouault]

Andrew,

> 
> Can someone please explain in some more detail what is meant by this
> statement in the description of OCTNewCoordinateTransformation() ?
> 
> "This will honour the axis order advertized by the source and target SRS,
> as well as their "data axis to SRS axis mapping". To have a behaviour
> similar to GDAL < 3.0, the OGR_CT_FORCE_TRADITIONAL_GIS_ORDER configuration
> option can be set to YES."
> 
> I'm trying to understand what the difference is from GDAL < 3.0 and how I
> might know that I'm seeing an issue and need to use the specified
> configuration option.

See https://trac.osgeo.org/gdal/wiki/rfc73_proj6_wkt2_srsbarn#Axisorderissues

Basically OGR_CT_FORCE_TRADITIONAL_GIS_ORDER forces 
SetAxisMappingStrategy(OAMS_TRADITIONAL_GIS_ORDER) on the copies of the source 
and target CRS used by OGRCoordinateTransformation

If you use EPSG:4326 or any geographic CRS imported from EPSG, as well as some 
projected CRS with a northing,easting order from EPSG, you'll see differences 
between GDAL < and >= 3.0. You may cope with that either by switching the 
order of coordinates you provide to / get from OGRCoordinateTransformation, or 
if you want to continue working with the order of coordinates you used to 
have, you can set OGR_CT_FORCE_TRADITIONAL_GIS_ORDER=YES, or call 
SetAxisMappingStrategy(OAMS_TRADITIONAL_GIS_ORDER) on the source & target CRS

Even

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