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 -- Spatialys - Geospatial professional services http://www.spatialys.com
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