The resolution of imagery obtained by fixed wing aircraft using digital cameras keeps getting higher and higher, not to mention the resolution obtained with low flying drones. Using the rule of thumb that control has to be 1/4 of the resolution, what do you tell a client who is doing 2.5 cm resolution imagery? That implies 0.625 cm (6 mm) accuracy for the ground control.
We do deformation surveys on dams where we get 3 mm accuracy over the structure, but that is using a high accuracy total station and redundant measurements (triangulateration), all with good visibility between stations. These imagery projects cover fairly large areas.
I tell the client 15 to 20 mm is the best we can do, and even that is difficult to get consistently. And how perfect does the target or ID have to be? That probably rules out using sidewalk corners. Only a paint stripe (or carefully painted target) could probably meet the criteria for sharpness. On the other hand, how does someone check the imagery and prove the control is not adequate if they only have the same capability (couple of cm's with GPS).
I realize there are probably some out there who will just tell the client they can get that accuracy, but I believe that is not economically achievable. If it is a low altitude drone flight, which covers a small area, then it is may be achievable by traversing, but still not easy. GPS? Not so much. Yes, you could do multiple long occupations, but that is not economically feasible.
It appears the accuracy requirements have surged ahead of what is achievable.
Wouldn't the vertical component of that be easily achievable with a digital level unless the site is so large as to preclude leveling.
