Hi Gordon, I own two drones. An Ebee RTK and a DJI Mavic Pro. I recently surveyed a road with my Ebee RTK. 7.5km x 200m of surface, 65m height, one flight direction (along the road). I did 7 flights. I set up 18 GCPs, one every 500 meters, approximately, or 3 points per kilometers. In hindsight, I guess that 5 points per kilometers would have been better. I obtained good results though. I also surveyed a bridge (500m x 150m of road, the bridge in the middle. 40m height, 5 flights in two directions along and across the road) with my MAVIC PRO. I used 5 GCPs. The minimum is 3 GCPs on a zone of this size. The accuracy between my Ebee RTK and my Mavic is not the same. I guess I woudn't need to set up GCPs if I had used my Ebee. But I also wouldn't use my Ebee on this small area. Using my Mavic on 7km of road would also be absolutely crasy. On a square or rectangle area of any size, the minimum is 3 GCPs. I prefer 5 GCPs. They must be well spread on the surface. On a corridor area, I work with 3 to 5GPs per kilometers and I try to not create a line (which is not easy to do in a corridor)...
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