UAS Mapping with DJI Phantom Pro 4 v2 elevation errors

[RjonesRIC]

Hello,

     We flew a quarry with our DJI Phantom Pro 4 v2 and I have been trying to create an accurate surface model to compare to what our surveyors did with their RTK units. Our cameras are in WGS84 and our ground control which was shot using our RTK units was shot in Missouri State Plane West Zone US Feet. I have brought in the images to photoscan and aligned them then I brought in our GCPs with the coordinate system they shot it in (yes I edited the state plane in photoscan to us feet) then I placed the markers on the painted X’s in the imagery. After I build the dense point cloud I create the DEM and export it out as a geotiff and within Civil 3D and create contours. The issue I’m having is the the elevation created from the UAS is on average 15’ off from the original survey. I was hoping someone could provide a detailed workflow to creating a DEM so the elevation can be as close as possible to the original survey. I am rather new with this process and I am trying to figure out a workflow that I can establish to meet client requirements. Any help would be much appreciated

[Alex737]

Unfortunately, without the exact coordinates of the image centers, it is impossible to obtain a normal result. Phantom 4 pro produces a beautiful but substandard result. It will be exact only on the reference points. Satisfied with the quality of Phantom 4 pro without PPK, the quality was not checked well enough, or the result was not checked at all. To obtain a qualitative result, use a copter with PPK. If you are used to working with Phantom 4 then go to www.topodrone.ru. With correct processing gives an accuracy of 5 cm.

[Hydro_Ydé]

You can have good results with P4P without PPK. How is you GCP distribution over the area ? I have one every 200m for quarries. First do global alignment with cameras and gcp checked, then optimize with gcp only. Have a close look on calibration result changes between the 2 phases, especially f.

[RjonesRIC]

We have really good coverage with GCPs horizontally and vertically. It just seams that after I process it and generate contours the elevation of the contour is 15’ lower than where we had a GCP. I would think that the elevation at that exact spot would match from the created DEM and GCP.

[GPC]

It sounds like you're making an error in the proccessing, there should be no issues like you're describing. Ignore Alex's post above, he's trying to sell you something. PPK is not needed and Agisoft does a great job.

I know the documentation isn't very good a few questions though:
Have you disabled the photos coordinates but unchecking them in the reference panel?  Do a low quality alignment, create mesh, pick control points, uncheck cameras, optimize and then do higher quality alignment and build. The issue may be the assigned default accuracy is still to optimistic and Agisoft of holding that weighting in the final adjustment



Also, you will see an error due to you working on orthometric height and the planes capturing either AGL or ellipsoid heights. Pay close attention to the coordinates in the reference panel.