Thank you.Thanks a lot to you @abant and congratulations for your WebApp.
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In all documents regulating the use of drones, it always refers to the height of the flight above ground level (AGL), so I use a generally accepted rule. And you are right, this altitude is set relative to the take-off point.
As for changes in the height of the earth's surface, this can be done, but the Elevation API of Google is not free unlike the Google Map.