<p>I have found the answer</p>
<p>Geopotential height of ground or water surface is available as a variable (<a href="https://rda.ucar.edu/datasets/ds084.1/#metadata/detailed.html?_do=y" rel="nofollow noreferrer">UCAR variable inventory</a>).
It is called 'orography' if you read the file with cfgrib. Somehow it did not appear in variable inventory when read with pygrib ...</p>
<p>Alternatively, I could have used <a href="https://unidata.github.io/MetPy/latest/api/generated/metpy.calc.geopotential_to_height.html?highlight=height#metpy.calc.geopotential_to_height" rel="nofollow noreferrer">MetPy</a>.</p>

CC BY-SA 4.0
2021-07-21T03:25:17.757Z
2021-07-21T03:25:17.757Z
raraki
<p>I would like to compare NOAA GFS (Global Forecast System) geopotential height from ASL and GTOPO30.</p>
<p>However, I cannot find vertical offset information in GFS geopotential height. Does anyone know where it is located?<br />
(And, I assume that GTOPO30 is in the geometric height from ASL ... is that correct?)</p>

<gfs><coordinate-system>
What is the vertical offset of geopotential height from ellipsoidal height in GFS?
raraki
# What is the vertical offset of geopotential height from ellipsoidal height in GFS?

(Asked by: username_0 on 2021-07-16)

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?)

**Tags:** gfs, coordinate-system


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## 1 answers

### Answer by username_0 on 2021-07-21. Score: 1 | Accepted answer

I have found the answer

Geopotential height of ground or water surface is available as a variable ([UCAR variable inventory](https://rda.ucar.edu/datasets/ds084.1/#metadata/detailed.html?_do=y)).
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](https://unidata.github.io/MetPy/latest/api/generated/metpy.calc.geopotential_to_height.html?highlight=height#metpy.calc.geopotential_to_height).


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