There was reports last year that the P4A/P went from storing GPS altitude to storing barometric relative altitude above ground relative to takeoff point. It appears to me that the altitude being used by Photoscan from photos taken by a Phantom 4 Adv/Pro is currently indeed the GPS altitude above sea level. So did it change back? Anyone with a P4A/P can also verify this if they use Photoscan. You can go into the reference pane and view the altitude values for each camera position. For a recent job, the ground elevation above sea level according to Google Earth Pro was 350m and Photoscan was showing 526m for camera position at that point. This was for a mission to fly 400 feet above ground level. Either Google or the Drone's GPS is way off though.
350m = 1150 feet. This is in the ballpark for the known elevation of the surrounding area (it's farmland.)
526m = 1725 feet
1725-1150 = 575 feet. That' 175 feet higher than the the mission altitude. The GPS altitude is actually supposed to be fairly accurate, I believe within 15 feet. Maybe this drone is faulty?
You can use this handy online EXIF viewer to see what's inside (
Jeffrey Friedl's Image Metadata Viewer ). In the XMP there is an Absolute altitude and a relative altitude. And in the EXIF there is GPS altitufde. The EXIF GPS altitude matches the Absolute altitude in the XMP and the relative altitude in the XMP is correct. The rumor before (may have been reality until a firmware update) was the relative altitude above ground from the barometer was stored in both fields. It is no longer. Check for yourself if you don't believe me. (One problem with the Internet, especially discussion forums, is how easily false information can spread because no one takes the time to verify for themsevles that what they are reading is true. Think "fake news" !)
Whether this was just a temporary change/glitch in the firmware some months back or if it was "fake news" I don't know.
My question is, does it matter which elevation type is used in photogrammetry processing as long as the drone's elevation above ground is maintained relatively consistent across all photos? Meaning if the drone goes over a valley and the altitude above ground because 100 feet higher, it seems with barometer above ground measurement, the drone would think the ground is 100 feet closer than it really is whereas GPS altitude above sea level would be consistent. It would seem to me you want a consistent altitude reading for all the photos for the entire job site despite your launch point. Seems this would be true using either measurement.
The big question to me is, if you are using absolute altitude in processing (above sea level), how can the drone know the absolute elevation of any point on the ground (without using GCPs)? Seems to me it can't. It can only identify relative elevation changes without GCPs. Now if the processing used altitude above ground, theoretically if you knew the base elevation of any point in the photo, you could therefore calculate the absolute ground altitude above sea level of any point in the photo by adding that to the elevation data output.
Admittedly, I'm on the fringe of my knowledge of photogrammetry in the context of generating ground elevation data so these are all semi-educated guesses on my part.