<div dir="ltr"><p class="MsoNormal" style="margin:0cm 0cm 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif">Hi,</p>

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<p class="MsoNormal" style="margin:0cm 0cm 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif">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. </p>

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<p class="MsoNormal" style="margin:0cm 0cm 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif">Would anyone be able to shed a bit of light on this or point
me in the right direction?</p>

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<p class="MsoNormal" style="margin:0cm 0cm 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif">Cheers,</p><p class="MsoNormal" style="margin:0cm 0cm 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif"><br></p><p class="MsoNormal" style="margin:0cm 0cm 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif">Sebastien</p></div>