Terrain Correction Output Elevation Offset


I recently used Snap to project an External DEM (RGE Alti) over an area of interest for which I had Sentinel-1 data.
To achieve this, I provided the DEM (as a GeoTIFF) during the Terrain Correction step of my process.

One issue I noticed was that the elevation values are modified between the input and the output for some reason. I don’t believe this is a resampling error: we’re talking about a 40m offset on average over the area of interest.

When I compared the output elevation values with the auto-downloaded SRTM, I saw that they were similar. However, the input elevation values fit what Google Earth shows for the area of interest.

What could cause this significant elevation difference? Is it just a matter of altitude of reference?
How would Snap know how to offset my external DEM?


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I found the answer.
The author in this post on the forum talked about EGM96.
This led me to check the effect of the “Apply Earth Gravitational Model” option in the Terrain-Correction parameters (only available when using an External DEM). The elevation offset did come from there.

There’s still an issue regarding why Google Earth doesn’t provide the same elevation values (despite supposedly also using EGM96), but it’s more for my curiosity than anything else.