How accurate is drone surveying? RTK and ground control points explained

The question I get most often is: how accurate is this, actually? The honest answer starts with a question back. Namely: what do you need the data for? Accuracy costs effort, and not every job needs the maximum.

Purpose first, technology second

If you want to see whether tiles are missing or where moss has taken hold, a clean photo will do. If an area is going to be calculated from those same images and someone orders material on that basis, it is a different matter. And if a result has to hold up in front of a loss adjuster or an authority, you also need proof of how accurate it actually is.

What RTK means

Ordinary GPS, the kind in your phone, only knows where it is to within a few metres. Fine for navigation, not for surveying. With RTK the drone continuously receives corrections from a fixed station on the ground during the flight. That means we know, to within a few centimetres, the position each individual image was taken from. That is half the battle, because the software later computes the model from exactly those positions.

What ground control points add

Ground control points are markers I place before the flight and measure precisely. They are unmistakable in the aerial images. They do two things: they pull the model into the right place and the right scale, and they serve as an independent check. Using a point I deliberately keep out of the computation, I can look afterwards at how far the result is off. That is a number I can quote you. It is the difference between "looks accurate" and "is demonstrably accurate".

Where we end up

With RTK and additional ground control points I work within a few centimetres on typical objects. How far you get always depends on the object too: open ground in even light is far more forgiving than a shaded courtyard between tall buildings. Flight altitude plays into it as well, because the closer the camera gets, the finer the image resolves.

The practical advice

Tell me beforehand what is going to happen with the data. If a tradesperson calculates from it, an authority checks it or an insurer has to accept it, I set the flight up accordingly. And if none of that applies, I save you the effort.