We don't have access to that information. We get either barometric relative altitude or GPS altitude from the DJI SDK and the aircraft. The GPS altitude has some really big altitude errors and basically can't be trusted.
Relative altitude is the best measurement that is available and we base our processing off of that. If you include a ground reference image with your upload, we use that to take the location of where the aircraft took off and get a value from the NASA SRTM database and use that as the offset for the rest of the images. Feel free to project it from the used WGS84 ellipsoid.
For more information on the SRTM data, read here: https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission
We cannot control what is reported by the firmware. We have no control over what values get written to the images. DJI seems to change it at will without telling anyone ahead of time or documenting that it happened. We have addressed the issue within Maps Made Easy by adding the workaround that is described above.
The method above works across multiple battery changes.