My stockpile is coming out too short.
I flew a 60 acre coal stockpile at 200’ AGL from the base of the pile. The pile was 60’ high in the middle. I used 70% front overlap and 60% side overlap. We are comparing the UAV flight survey to a traditional survey done the day before. The pile change was negligible between the two days. I’m flying a DJI Phantom 3 Pro using Drone Deploy for the flight planning and flying.
The processing went great! Or, so I thought. Once we got the results from the ‘real’ survey (done with GPS and Laser scanners) my pile is about 5’ too short resulting in a huge (9%) volume difference from the official survey. The GCP’s tied in very nicely and everything at that lower elevation matches near perfectly with the other survey. As we climb up the pile, however, the discrepancy begins and gets worse as we go up. I used 5 GCPs around the perimeter of the pile.
What causes this and how can I prevent it in the future? More overlap? Getting GCPs on the pile itself would be a safety issue and we would prefer not to do that.