I have asked the same and this was the reply:
Supposedly if you place a geotiff in the srtm directory Mission Planner will use it.
This was a solution from Michael Oborne.
Apparently Mission Planner uses geotiff format for elevation data.
all the video is really doing is
1. load elevation data in globalmapper
2. check projection is wgs84
3. export elevation data as 16/32bit geotiff
4. place geotiff in srtm folder.
5. test that the data is loading from the geotiff.
Apparently it uses WGS84
Can you let me know if this works.
Make sure you have the latest Mission Planner.