I need to convert the 1/3 arc second DEM files from the NED into WGS84 ellipsoid heights. The documentation says that the NED is referenced to NAVD88. I've tried using gdalwarp as follows:

gdalwarp imgn45w124_13.img imgn45w124_13_wgs84.tif -t_srs "+proj=longlat +ellps=WGS84"

But this doesn't change the heights at all, and they are still off. What is the correct way to do this?

I've also tried the following without success just incase GDAL isn't able to read in the vertical datum info.

gdalwarp imgn45w124_13.img imgn45w124_13_wgs84.tif -t_srs "+proj=longlat +datum=WGS84" -s_srs "+proj=longlat +datum=NAVD88"
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    Guessing based on what I know of PROJ.4 and GDAL. You need to make sure that the geoid files are available (look for .gtx files), probably one called gxxxx as that should be a US geoid model like g2009conus.gtx. You may see egmxx but that's a worldwide one. In the proj string, include +geoidgrids=name-of-file. Do a web search for gdal and geoid. – mkennedy May 28 '16 at 0:19

I didn't figure out how to make it work with the gdalwarp, but I downloaded VDatum and did a single point coversion. After which it identified that I needed to use the geoid12b. Then I found in the VDatum directory the geoid12b/*.gtx files. Opened the NED file in gdal and the geoid file in gdal and created a matching projection of the geoid into the NED and added it in. The NED now matches my InSAR data nearly perfectly.

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  • Can you give any details on the steps you used to do this in gdal? – corvus Aug 21 '18 at 5:49
  • Ah, unfortunately, I don't have access to the code I wrote for that anymore. – Craig Aug 23 '18 at 3:48
  • I found out that you can download datum information from NOAA that I was able to use for this. (ngs.noaa.gov/GEOID/GEOID12B/GEOID12B_CONUS.shtml) You can also run VDatum online by uploading a CSV of points, though the number of points is limited. (vdatum.noaa.gov/vdatumweb/#ASCII) Both methods are a bit involved and require parsing text files into spatial data, but it is doable & as far as I can tell this is the only good source for such data. – corvus Aug 23 '18 at 22:53

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