The short answer to this question is that the elevation in Propeller is based on the elevation in the data you send to us. Elevation in, elevation out!
- Geotags only with takeoff elevation: If your drone is set up to record the height above its takeoff point in geotags, the dataset will usually show an elevation on the ground mostly just above or below zero. Even if you're flying at the top of Mt Everest, the number you'll see will be around 0.
- Geotags with GPS elevation: If your drone is recording the GPS elevation above 'mean sea level', the data will be roughly aligned with the height of terrain in this area above the WGS84 ellipsoid used by GPS. However, GPS elevation data is accurate within only 20m or so, and there may be very significant differences from a measured ground point. The WGS84 ellipsoid is also very different to the actual sea levels in many places by over 50m.
- Ground Control Points or AeroPoints: if you use ground control points to make your data more accurate, you'll see an elevation based on the height datum used for the ground control. Most surveying data uses a geoid height datum - a correction that accounts for the lumpiness of the planet and gives you an elevation much closer to what the sea level would be at a given location.
You can use the CRS setting in the Settings menu to choose a different co-ordinate system. Different ellipsoids or datums can affect the height displayed.
You can also use the settings in the top right to switch between metric and imperial units.