What is the vertical offset of geopotential height from ellipsoidal height in GFS?

Question

I would like to compare NOAA GFS (Global Forecast System) geopotential height from ASL and GTOPO30.
However, I cannot find vertical offset information in GFS geopotential height. Does anyone know where it is located?
(And, I assume that GTOPO30 is in the geometric height from ASL ... is that correct?)

raraki · Accepted Answer

I have found the answer
Geopotential height of ground or water surface is available as a variable (UCAR variable inventory).
It is called 'orography' if you read the file with cfgrib. Somehow it did not appear in variable inventory when read with pygrib ...
Alternatively, I could have used MetPy.

What is the vertical offset of geopotential height from ellipsoidal height in GFS?

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