How to optimise your GCP

Ground Control Points (GCP) are the most important element of drone(UAVs) surveys, as they provide accuracy. Drones with RTK * or PPK ** are great in the air, but their accuracy is not automatically transferred to the captured image of the earth's surface. In essence, GCPs reduce only the magnitude of error from meters to centimeters.

Imagine GCP as weights fixing the area of the construction site like stones on a tarpaulin. If there are no weights on the edge, the tarpaulin will bend. Also, a few weights in the middle are needed to keep it taut and unable to be blown by the wind into an air bubble.

Visibility must also be taken into account. Place all your GCPs in open spaces with a clear view of the sky. Avoid placing them next to trees, near fences, buildings or other obstacles.

Frequently asked questions:

1. Should I put more GCPs in the places where I need more accuracy?

The short answer is no. Moreover, the accumulation of more points will not only not increase the accuracy, but may worsen it. Let's look at an example to see how things actually work. It is logical to think that more points at one end will give us more accuracy there. The distribution of the points of the scheme has such a presumption. But this common mistake leads to disappointing results. Note that the points does not cover the entire site; there are none at the highest point; and that some of the GCPs is grouped together around important sections. Badly covered sections of markers actually pull well-covered sections in a way that makes the whole model unusable. This survey cannot be corrected correctly, resulting in inaccuracies.

2. Which is the best destribution?

What you need is an optimal distribution of GCPs throughout the subjected area. This is crucial, much more important than gathering them together. Create a shape that limits the area of your object, as well as cover the environment as evenly as possible. Try to cover the highest and lowest parts. In places without GCPs, it will be very difficult to capture the photos from the survey in this area.

3. What is the best example of placing GCPs on the ground?

Here we see the same object, but this time with evenly distributed tubing. Note that with this distribution, you can easily connect the dots without removing the pencil from the sheet.

When planning the locations of the points, remember that the tubing should be located on the perimeter and in the middle. You don't want the imaginary tarpaulin you anchor to wrinkle too much, or worse, to sway in the wind. These cases would make the data from your survey highly inaccurate or even unusable.

* / RTK - Real-time kinematic - technology for real-time error correction. In general, during the flight, with the help of the RTK module, the drone georeferences exactly the image (with geographical coordinates X, Y, Z). An active base station sends correction data to the drone. The drone's on-board GPS module combines this information with its own observations, and pinpoints the location relative to the base station at any time in real time.

** / PPK - Post-processing kinematic - UAV marks with geographical coordinates X, Y, Z each image based on the built-in GPS device. While this is happening, the base station also records position information, but with much more accurate triangulation. Upon completion of the flight, these two sets of GPS data are combined using the time stamp of the photo. The initial, less accurate on-board GPS data is then corrected by giving accurate geotags for the images.

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5Д СИМ ЕООД, Огнян Ралчев