Wrong altitude data from drone location coordinates

[anna.i]

Hello everybody!

I just recently started to use the Phantom 4 pro v.2 to generate high resolution digital elevation models. I'm just trying different processing workflows so far using Agisoft Metashape, and even without implementing GCPs, the global location accuracy of the output derived from the camera postions is quite good. In a next step, I wanted to implement GCPs I measured using a differential GNSS (so the accuracy is < 2cm in x/y/z) and discovered, that the altitude of my camera positions is completely wrong - the data generated from the images is ~177m lower then the actual surface. I know that the elevation of the GCPs in right, I compared it with the coarse DEM provided by the state. As I measured the GCPs originally in the local UTM-system (WGS84 UTM 33 N, EPSG 32633) I reprojected it to WGS 84 global (EPSG 4326). I read something about different altitude types (altitude above sea level, altitude above ellipsoid), but even considering that, 177m difference seems too much (when using an online tool to recalculate height data, it shows differences of about 46m between ellipsoidal height and mean sea level).

Did anybody experience similar problems and can anybody tell me something about that issue? The added image illustrates my problem.

Thanks a lot and cheers from Vienna!


Anna

[Alexey Pasumansky]

Hello Anna,

Metashape is reading the altitude information from the EXIF GPS tag of the source photos. So it seems to be an issue related to the drone firmware. You can, however, add the fixed offset to all the cameras in the Reference pane to correct (at least to some point) the problem. See the related script:
https://github.com/agisoft-llc/metashape-scripts/blob/master/src/add_altitude_to_reference.py

[anna.i]

Thanks a lot Alexey for the quick response and the link to the script, that's at least an elegant workaround.
 I'll perform an firmware update before I go out the next time and report on that issue.

[GrippaH]

Out of curiosity, i have the same problem, and the height difference is around 46m. How do you run the script please. Complete noob here so apologies if being dumb

[RHenriques]

Be aware that most UAV's use WGS84 with ellipsoidal height. To have errors around 2-3 meters, compatible with local altimetric datums, you have to correct the UAV height with a geoidal model for your area. To have sub-metrical precision, you have to use Ground Control Points obtained with GNSS.

[GrippaH]

Thank you RHenriques. I have Reach RS+ so will try with that.