Last thing I'll add to support my point.

I emailed DJI and here is what they emailed me back.

It went through their normal Support and to their "engineers" and took a week to get.

"Thanks for your patiently waiting.

Here are the answers to the drones you were asking:

Mavic 2 Enterprise Advanced - Ellipsoid

Mavic 2 Enterprise/Zoom - relative altitude

Phantom 4 Pro V2 - relative altitude

Phantom 4 RTK - Ellipsoid

Matrice 300 RTK - Ellipsoid

Aircraft without RTK (including consumer aircraft) only use relative altitude or TOF altitude. When the aircraft actually uses the altitude, it is calculated based on the barometer itself. It is based on the altitude, not ellipsoid height."

I found this to be spot on except for my

Phantom 4 Pro V2.

Here is the results of the test I did for a paper I wrote on Public Safety using drones to produce maps without GCPs and/or RTK and then with GCPs.

4 drones were tested with three of them being DJI. A Phantom 4 RTK was also thrown in to show how when you want to map, use the correct tool. The altitudes flown varied due to attempting to get to a desired GSD. (Some GSD's were incorrectly predicted by DJI Pilot.

Drone | Set Altitude (ft) | Set Altitude (m) | Rel Altitude | Absolute Altitude | GPS Altitude |

Mavic 2 Enterprise Advanced | 177 | 53.95 | 53.85 | 28.17 | 28.1 |

Mavic 2 Enterprise | 210 | 64 | 63.9 | 67.67 | 67 |

Phantom 4 Pro V2 | 182 | 55.47 | 56.2 | 31.53 | 31.5 |

| | | | | |

| | | | | |

Ellipsoid height in area = -32 | | | | | |

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| | | | | |

| | | | | |

Geoid Height in area= -34 | | | | | |

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H = h - n | | | | | |

Orthometric Height = Ellipsoid height - Geoid Height | | | | | |

| | | | | |

2m average Orthometric height in area | | | | | |

Set altitude is what I commanded DJI Pilot and GS Pro to fly to attempt the desired GSD.

First aircraft

Mavic 2 Enterprise Advanced

It flew at 53.95m which matches up real nice with the Rel Altitude of 53.85m.

Absolute Altitude was 28.17m. There is no way possible this is referring to Above Sea Level. ASL would be around 52m with the ground being 2m ASL. Further evidence is that the ellipsoid height in this area is -32m. Add your flight height of 53.85 to this and you get 21.85m which is close enough to 28.17 m to be the inaccuracy of a consumer grade GPS on this drone. It makes much more sense than saying this is ASL. And it also matches what DJI said about reporting as above ellipsoid.

For this to be reported as Above Sea Level it would be off by: around 24m

For this to reported as above ellipsoid it is off by: around 6m

Second aircraft

Mavic 2 Enterprise

It flew at 64 meters as set in DJI Pilot which is almost spot on with the Rel Altitude of 63.9m

Absolute altitude is listed as 67.67m. For this to be ASL in this location (2m ASL) it should be around 62m. This is close.

Lets look at if this could be ellipsoid. Add your flight height of 53.85 to -32m (ellipsoid) and you get 31.9 meters. This is again too great of a difference in what it should be. This matches up with what DJI emailed me back again considering the consumer grade GPS on this drone.

For this to be reported as Above Sea Level it would be off by around: 6m

For this to be reported as above the ellipsoid it is off by: around 32m

Last but not least, let's throw in the wild card of this test.

Phantom 4 Pro V2
DJI said that this would not be reported as ellipsoid heights.

It flew at a commanded 55.47m which is pretty close to our 56.2m Rel Altitude.

If this was ASL this should be around 54m for Absolute Altitude. We are not even close with 31.53 being in our metadata.

Okay, lets see if this makes more sense to be reported as ellipsoid. Add our flight height of 56.2m to our ellipsoid value of -32 and we get 24.2.

For this to be reported as Above Sea Level it would be off by around: 22m

For this to be reported as ellipsoid it would be off by around: 7m

Let's add something more concrete.

This

Phantom 4 Pro V2 had a PPK kit on it and its dataset was run both with the original non-corrected images and then the PPK tagged images.

The PPK corrected image (Base Station setup on a low Standard Deviation NGS monument that is also a part of hybrid Geoid 18 and used to constrain the new GRAVD gravity model) had a corrected altitude in ellipsoid as 26.09m which matches up decently for our 31.53m when we consider the consumer grade GPS.

Unfortunately the

Phantom 4 Pro V2 does not match what I was told by DJI. But it does fit better as an ellipsoid height and when processed in Agisoft with the software expecting the image's altitudes to be above ellipsoid, it processed with a vertical error of 4m.

Summary of RMSE with elevations being reported as follows:

Mavic 2 Enterprise Advanced: Above Ellipsoid

Mavic 2 Enterprise (Zoom): I mass edited the metadata altitude to 32m for all images to reflect the flight height above the -32 ellipsoid, then had Agisoft expect elevations to be referred to ellipsoid

Phantom 4 Pro V2: Above Ellipsoid

Please ignore the other aircraft tested as I copied and pasted from my paper.

sUAS | X | Y | Z | Total |

Phantom 4 RTK Double Grid (Oblique 60*) | 0.77 cm | 0.50 cm | 1.71 cm | 1.94 cm |

Phantom 4 RTK Single Grid (1 Diagonal Line Oblique) No GCPs | 0.66 cm | 0.50 cm | 3.08 cm | 3.19 cm |

Phantom 4 Pro V2 PPK | 2.11 cm | 1.41 cm | 10.67 cm | 10.97 cm |

Anafi Thermal | 0.36 m | 1.85 m | 0.34 m | 1.92 m |

Mavic 2 Enterprise (Zoom) | 1.51 m | 1.51 m | 0.74 m | 2.26 m |

Phantom 4 Pro V2 | 0.21 m | 3.23 m | 4.20 m | 5.30 m |

Mavic 2 Enterprise Adv. | 4.38 m | 0.86 m | 4.52 m | 6.35 m |

Different DJI drones report their elevations differently and it is not always ASL or ellipsoid as reported by both DJI and this one of several tests I did. Pix4D actually caught the

Mavic 2 Enterprise Advanced as being above ellipsoid early on when I was suffering from incorrect elevations. I then emailed DJI and go the above response.