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<p class="MsoNormal"><span style="font-size:11.0pt">Hello all, I’ve recently learned I’ve been using my elevation dataset wrong and could use some expert tips. I’m using ASTERv3 as my DEM source and that’s defined relative to EGM96. I’ve been using OSSIM for
orthorectification and it turns out I configured that correctly by accident.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I tried building a VRT that summed my ASTER and EGM96 VRTs but it seems that the only way to do this was using embedded python which feels like overkill (not to mention my python version selection issues).
I’d like to avoid this method.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I found a few references to EPSG:4326+5773 but couldn’t get it working in any of the combinations I tried.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I set the SRS during VRT build and it produces a VRT with three axes which seemed off; gdallocationinfo on the VRT returns the ASTER height uncorrected. Looks like this was the wrong way to do it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Neither “gdaltransform -s_srs EPSG:4326 -t_srs EPSG:4326+5773” nor the reverse gave the EGM96 offset. gdalsrsinfo gives this proj string: “+proj=longlat +datum=WGS84 +geoidgrids=us_nga_egm96_15.tif +geoid_crs=WGS84
+vunits=m +no_defs” so I tried that (with the full path to the EGM file) as well and didn’t get anywhere.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I found an old ticket comment of Even’s (hi
</span><span style="font-size:11.0pt;font-family:"Apple Color Emoji"">😊</span><span style="font-size:11.0pt">) that seems to suggest what I’m trying should work
<a href="https://trac.osgeo.org/gdal/ticket/5648#comment:1">https://trac.osgeo.org/gdal/ticket/5648#comment:1</a> but I couldn’t get debug logging working with CPL_DEBUG to see if anything was failing in the backend. I’m using the 3.10.2 ubuntu full container,
so there shouldn’t be any proj file lookup issues and none were printed.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Alternatively, offsetting the ASTER tiles manually includes an int16->float32 conversion so the dataset doubles in size which I don’t love, but disk is cheap. I could truncate for +-0.5m error, but I’d rather
have a larger dataset than a less correct one. I’m curious if anyone has thoughts on supersmapling EGM96 the ~900 times to match ASTER’s resolution as it feels like such a jump would have important nuances. In my tests so far I’ve just used bilinear interpolation.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Eager to hear what others think!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Will<o:p></o:p></span></p>
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