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My stockpile is 'squooshed' (flat)

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. 

Thanks

Lyle, can you message me the details of your project? I am interested in what may have caused this, do you have access to the original survey data?

Lyle,

I am having the same problem you expressed with your stockpile accuracies. Did you find a solution?

We determined it was a lack of enough GCP. My GCPs were on the ends of the pile. It was an oval shaped pile running north/south. I had 3 points on the north end and two on the south. I added two more on the West side and Two more on the East side and the pile came up nearly perfect with the official survey. The best explanation I heard was pretend each GCP is a pole. Now throw a table cloth over those poles. It’s going to sag in the middle. The more poles, the less the table cloth sags.

I was lucky to have some visible points that were identified with previous surveys. Otherwise I would have had to re survey those points.

BTW, we ended up within 0.4% of the official, traditional survey done with GPS and Laser scanning.

It’s worth mentioning, too, that I should have either flown higher, or with more overlap. I’ve chosen to fly with 80/80 overlap on recent projects. With the coal pile being 60’ high, that gave me an effective 140’ AGL when flying over the top of the pile which reduced my overlap significantly. So I barely had enough overlap at the top of the pile when flying at 70/60 percent overlap.