[gdal-dev] UTM local correction for height and distance-to-reference-meridian ?

[Javier Jimenez Shaw]

If you don't want to measure an ogr geometry but produce for instance a
geotiff in a LDP (low distortion projection), you can just convert to a
transverse mercator (without "universal") centred in your area of interest.
Makeing a wkt by hand is very easy.

In case you want to consider also the distortion due to height, I recommend
either compute a scale factor for your projection (as explained somehow in
https://geodesy.noaa.gov/web/science_edu/webinar_series/changes-afoot-after-2022.shtml
) or compute it yourself.

For the projection distortion factor (not including height) there is
something in PROJ:
https://proj.org/en/stable/apps/proj.html#cmdoption-proj-S

As a curiosity or helper there is a plugin for QGIS to visualize the
factors: https://plugins.qgis.org/plugins/proj_factors_redux/ (Thanks
Johannes)


On Tue, 21 Jan 2025 at 11:45, Even Rouault via gdal-dev <
gdal-dev at lists.osgeo.org> wrote:

>
> Le 21/01/2025 à 11:37, Thomas Knudsen via gdal-dev a écrit :
>
> If you plan to transform to a different system anyway, I would definitely
> prefer going to geographical coordinates, and computing the length of the
> geodesic between the two points.
>
> actually no need to do the explicit conversion to geographical
> coordinates, if the appropriate SRS is attached to your geometry,
> OGRGeometry::get_GeodesicLength() will do the right thing:
> https://gdal.org/en/stable/api/ogrgeometry_cpp.html#_CPPv4NK13OGRLineString18get_GeodesicLengthEPK19OGRSpatialReference
>
> Python API:
> https://gdal.org/en/stable/api/python/vector_api.html#osgeo.ogr.Geometry.GeodesicLength
>
> Python examples:
> https://github.com/OSGeo/gdal/blob/master/autotest/ogr/ogr_geom.py#L4560
>
> -- http://www.spatialys.com
> My software is free, but my time generally not.
>
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>
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