Skyveyor112
lvl.4
United States
Offline
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I am still collecting and analyzing data but thought I would share my preliminary thoughts.
Yesterday, we flew two terrain awareness missions over the same area from the same home point. The vertical offset was 100m above the DSM. The elevation data in the DSM was orthometric height. No base station was used for either flight. For the first flight, we were not connected to an NTRIP server. The second flight we were, and all photos were RTK.
From the EXIF/XMP data from the non RTK flight the average relative altitude was 191 m above the ground, which is 91m higher than it should have been. Due to experience with the P4P, I suspect this error is related to barometric pressure. When the barometric pressure is anything other then standard 29.92, the P4P EXIF encoded altitude is off 10ft for every 0.01 inHG. However the P4P flies a relative altitude based on the pressure difference between 29.92 and ambient pressure, which leads to fairly consistent relative altitude, even with terrain following. 91 m is approximately 301 ft, which equates to .3013 inHG. Adding this to 29.92 equals 30.22. The local airport (about 10 miles away from the home point) reported altimeter reading of 30.22 for the time of flight. Stunning coincidence?
The second flight (with rtk) EXIF/XMP data indicated average relative altitude was 127 m, 27 m higher than the offset.
Initial thoughts:
1) The Phantom RTk doesn't consider home point pressure variation from 29.92
2) The Phantom RTK relative altitude doesn't account for non standard altimeters settings and flies high or low based on ambient barometric pressure. On high pressure days the drone flies higher than desired , leading to higher GSD and much greater overlap. On low pressure days overlap is less than planned, which may be insufficient for photogrammetry. And significantly could lead to the drone crashing into the terrain.
3. When in RTK mode, the drone appears to offset or correct for barometric pressure. I am assuming the Phantom RTK still flies barometric altitudes and not GPS.
4, The GEOID separation in my area is 33 m, which is only 6m different from the average relative altitude with RTK data, which may be attributable to orthometric height in the DSM.
I know its not good science to draw conclusions from limited data. But that is all I have and I am eager to share with this community to hear their thoughts and other explanations.
We are headed back to the field today to do two more flight. One RTK one non RTK, both with ellipsoidal elevation data in the DSM.
I'd like to thank Phil Harvey for making ExifToolavailable to the public. Without it I could not have extracted relative altitute from the XMP data. |
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