[gdal-dev] gdalwarp vertical datum shift question

[Mark Warren]

Hi, I have a question regarding gdalwarp.

We would like to update gdalwarp to be able to be used for DEM files
such that a vertical datum shift can be added to the files, as well as
the horizontal warp. Especially we would want to be able to use the
geoidgrids vertical grid shift option.

As an experiment I have edited a line in the gdalwarpkernal.cpp file in
the function:

static CPLErr GWKNearestNoMasksFloat( GDALWarpKernel *poWK )

((float *)poWK->papabyDstImage[iBand])[iDstOffset] =
    ((float *)poWK->papabySrcImage[iBand])[iSrcOffset]-padfZ[iDstX];

such that instead of assigning the src image value, it assigns that
value with the Z coordinate transform value subtracted. I have checked
this with some warp transformations from WGS84 to OSGB36 and it appears
to work to a certain accuracy (within 10's of cm).

One thing I'm unsure of is whether the fact that the transformation from
destination grid to source grid with the Z value =0 is going to be
inaccurate or not. Should the "proper" way be to assign this the actual
Z value from the source file prior to calculating the transformed
values? This seems that it may be difficult to do as it is using a
backwards transformation (dest to src) so we don't know the src pixel
that relates to the dest pixel until after the transformation.

Another obvious issue would be that we would only want this to happen for 
images with a vertical datum associated (like DEMs) and not for other 
georeferenced imagery that we may want to warp.

So before we go down this route too far (since a lot of changes may be
needed to the code) - does anyone think that this is a feasible idea or
have any other suggestions (other than using a GIS :) )?

Thanks,
Mark

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