[gdal-dev] Interpolation used for egm96_15.gtx when using RPC_DEM_APPLY_VDATUM_SHIFT

[Even Rouault]

Yann-Sebastien,

> I spent a bit of time learning about vertical datum shifts when performing
> RPC coordinate transformations using a DEM this week, and I was curious
> about how the egm96_15.gtx file is interpolated. I was able to find in
> gdal_rpc.cpp that the required vdatum shift is calculated using an OGR
> Coordinate Transformation between the DEM spatial reference and a wgs84
> ellipsoid spatial reference, but I’m not familiar enough with OGR/PROJ to
> understand exactly how the gtx file is sampled.

To start with, you need the the dataset pointed by RPC_DEM is a compoundCRS with a 
vertical CRS properly defined and an existing transformation between that vertical CRS 
and WGS84, so if it is something like EPSG:XXXX+5773, that will be fine.
Interpolation of the geoid grid is done by PROJ itself, and this is bilinear interpolation.

Even

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