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?