Number of GCPs, Overlap / Sidelap and two drones for large area topo?


#1

I need to generate a topographic survey of a 700 acre site (basically rectangular shape) where the maximum vertical accuracy error will be one foot (12 inches - 30cm). My understanding is vertical accuracy should be within 3X GSD, so that means my GSD needs to be 10cm (4 inches) or better. So I will be flying at max AGL (400 feet). There’s going to be fairly large elevation changes over that large of an area probably as much as 200 feet during some flights (maybe only 25 feet on other sections). The terrain is hilly but without water/trees/crops - somewhat desert like and sparse structures. The absolute accuracy on the horizontal plane also can have an error up to 12 inches. Given these requirements, what would be a good number of GCPs to use (how do you determine that figure?) and what would be a good minimum sidelap / overlap?

Also wondering could two people using two identical drones, such as a DJI Phantom 4, could be used to save time or would possible variations in the altitude measurements between the two drones possibly introduce large errors in the resulting work product?


#2

Hello Jeff,

Thanks for reaching out. Impressive project. Hope it turns out well! So with the number of GCPs, I would recommend 5, but you can place up to 10 GCPs in a given map. More information on that can be found here This article will give best practices when using GCPs.

A good number for Side/Frontal overlap would be about 75/75. I would bump this up to 80/80 personally. However, this may increase the flight time thus increasing the number of batteries you would need to complete the flight.

You may use two drones to map the same area. You would just need to make sure that your photos are geotagged and the file numbers are not the same to avoid the risk of overwriting your data. I would set the altitude the same for both flights. The accuracy will be better if you flew with one drone but I believe the variance will not cause drastic errors.

Thanks and let me know if you have further questions on this project. @consultant