For the same spot, the takeoff elevation recorded from missions over a 2 week period showed a 500’ variation. This was seen over the period from 10/26/2017 to 11/9/2017 on a DJI Phantom 4 Pro drone. Thus all of the altitudes recorded for all of the missions over this period will show the same 500’ variation.
I have read that DJI estimates the altitude of the P4P from its on-board barometer and my observations are consistent with this. The graph below of the takeoff elevation vs the local barometer reading shows that they are strongly correlated. The actual elevation is 1000’ so the DJI P4P drone is close much of the time.
The barometer range of 0.48" Hg in the graph above is equivalent to a 453’ elevation change and this is in the same ballpark as the 496’ elevation change recorded in the photos.
If the recorded altitude is barometer based, then the P4P should fly very level as the barometric pressure typically varies very little over a 10 min mission. This should lead to very accurate relative elevation measurements in high-quality regions of the map (those with the best overlaps) and my 2D map results are consistent with this; I can see the 1/5" drop per foot carefully built into my driveway over a 124’ distance as shown in the 2 segments below:
Unfortunately the severe variation in the absolute altitude can cause serious problems when trying to add more photos later on in order to improve the map quality. Obviously GCP’s could fix this problem but there is a much simpler first-order fix: Simply add an offset to all measured altitudes so that the start location elevation is always the same. This can be done to the photos before processing but it could be done more easily in DroneDeploy while processing the pictures.
So my ask of the DroneDeploy developers is:
Could you please add an elevation offset option to the form where photos are uploaded and echo its value back on the results form?
Two ways to calculate the offset are:
- Offset = Actual Elevation of Takeoff Point - Barometer-based Altitude recorded in the first photo at the takeoff location
Or after the photos are processed into maps:
- Offset = Actual Elevation of Takeoff Point - Measured Elevation of Takeoff Point on 2D map
Obviously the second way requires double processing of the photos but should be more accurate.
Without correction, adding pictures taken with a significant delay after your primary set may degrade rather than improve the Map quality. At the very least, compare the value of the GPSAltitude in the first photo at the takeoff location for the 2 sets of photos. To check the GPSAltitude you can right-click on the picture, select Properties > Details and scroll down to find GPSAltitude. Or use the exiftool by Phil Harvey.
Remember every time the local barometer reading changes, the recorded elevation of the takeoff location (and all other locations) will change. Each +/-0.01" Hg change in barometer reading causes a -/+10’ change in the recorded takeoff elevation. And most locations see a 1" Hg change over the course of a year. This is a 1000’ variation so beware. My P4P has already recorded a 496’ variation in just a 2 week period!
But maybe other drones are different and do not record an altitude estimated from an on-board barometer. What does your drone do? It would be great to hear about drones that do this better.