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Accuracy & RTK

How accurate will my model be?

You can measure lengths and areas to the centimetre without RTK. RTK makes sure the model also sits in the right place. Which jobs call for that.

Two kinds of accuracy

Relative accuracy tells you how precisely you can measure a distance in the model: the ridge length, a roof surface, the width of a road. With a well-planned flight it is already very good without RTK: typically around 2 cm, and roughly 1 cm with a high-resolution survey camera such as the one on the DJI Matrice 4E. That difference comes from the camera, not from RTK.

Absolute accuracy tells you how precisely the model sits in the coordinate system. Picture a road junction surveyed by drone. You take the coordinate for a new traffic light from the model and have it built exactly there. Without RTK the light may end up 20 to 30 cm off, almost on the pavement. With RTK it is 2 to 3 cm.

Left: a distance on the object. Right: dashed, the target position; orange, where the model puts the house.

What RTK does

RTK stands for Real Time Kinematic. Every drone works out its position via GNSS, meaning GPS, Galileo, GLONASS and BeiDou. That is plenty for flying, but the position is only good to one to three metres. With RTK the drone also uses correction data from a reference station and gets down to 2 to 4 cm.

The key point: RTK improves only the position of the drone. Camera, image quality and flight planning stay as they are. Nor can the drone generate the correction data itself. It comes from outside. Where from, and how to set it up, is covered in Setting up RTK.

How precisely the drone knows where it is. The figures apply to the drone's position, not to distances in the model.

Do you need RTK?

Five typical jobs, from “fine without” to “impossible without”.

Roofs of houses and apartment blocks: worth considering

For lengths and areas a DJI Mini or Air without RTK is enough. Roof pitch is different: without RTK the building can sit slightly tilted in the model, even though it stands straight in reality. The scale is right, but both roof pitches are off by the same amount. RTK prevents that.

The same gable roof: tilted without RTK, straight with it. The angles still add up to the same total, but they are split wrongly.

The tilt shows up here as well. On top of that, distortion becomes more likely over large distances without RTK: flat surfaces come out slightly wavy or curved in the model. That stays the exception, but you cannot predict it. RTK-capable drones also offer the area route, which helps prevent such errors (see flight planning).

Over the full length of a hall, a flat roof can sag without RTK.

Architects and planning offices: necessary

As soon as you work with others, you need georeferenced data: the model has to sit in the right place in the world coordinate system. If you lay a partner’s CAD model over the point cloud to compare design and as-built, the building must not appear shifted because of a measurement error. Without RTK that is hardly possible, whatever the size of the object.

Dashed, the CAD model; dotted, the point cloud from the flight.

Landfills and earthworks: a must

For cut-and-fill volumes, two flights perhaps a quarter apart have to line up exactly. If one data set sits just 10 cm too high, you quickly get many cubic metres of cut or fill that never happened.

Grey, the first flight; orange, the second. Without RTK a height offset turns into phantom volume.

Smart 3D Capture and similar modes: mandatory

With Smart 3D Capture and other advanced modes the drone navigates automatically in tight spaces. It therefore checks for an RTK connection before take-off and will not start the mission without one.

RTK is not everything

Camera resolution, altitude, overlap and flight strategy matter just as much. Quality comes from the flight, RTK comes on top. More under flight planning.

In short

For measuring up, quoting and documentation you get a long way without RTK. As soon as the model has to match other data, volumes are compared over time, or the flight mode demands it, there is no way around RTK.

Which drones support RTK, where the correction data comes from and what to do when you cannot get a Fix: Setting up RTK.

Common questions

How accurate is a 3D model made from drone photos?
Distances and areas in the model are typically accurate to about 2 cm, and to roughly 1 cm with a high-resolution survey camera such as the one on the DJI Matrice 4E. That is the relative accuracy. How precisely the model sits in the coordinate system is a second question: 20 to 30 cm without RTK, 2 to 3 cm with RTK.
What is the difference between relative and absolute accuracy?
Relative accuracy tells you how precisely you measure a distance in the model, such as a ridge length. Absolute accuracy tells you how precisely the model sits in its place in the coordinate system. For measuring up and quoting the first one counts; as soon as the model has to match plans or other surveys, the second.
Does RTK make my model more accurate?
It makes the position more accurate, not the distances. RTK only improves the position of the drone, from one to three metres down to 2 to 4 cm. Whether a distance in the model is right to 2 or to 1 cm is down to the camera. On top of that, RTK keeps the model from tilting or warping over large areas.
Do I need RTK to measure a roof?
Not for lengths and areas; a DJI Mini or Air is enough for those. RTK is worth having if you read the roof pitch from the model: without it the house can sit slightly tilted in the model, and both halves of the roof then show the wrong angle.
When is RTK a must?
When the model has to line up, georeferenced, with other data, such as a project partner's CAD model. When you compare volumes from two flights, for example on landfills or in earthworks. And when the flight mode demands it: Smart 3D Capture will not even start without RTK.
What else does accuracy depend on?
On camera resolution, flight altitude, photo overlap and the flight pattern. Quality comes from the flight, RTK comes on top. The values for each pattern are in the flight planning chapter.