Ellipsoidal to orthometric altitudes for geoid model

The geoid models have sometimes a hard to decode format, such as IGN .mnt files https://geodesie.ign.fr/index.php?page=grilles

For now, I’ve just found third parties software that can do the format conversion, even if this could be doable with a bash or python script.

Such files can be converted to ASCII XYZ format with the demo version of Hydromagic, an hydrographic surveying software. Here is how to do :