What is a GCP?

GCP stands for Ground Control Point. This is used as a reference and control point for a drone survey.

GCPs can be either sprayed with paint or placed using fixed targets. Their position is crucial for the quality of the survey.

As shown in the photo, a GCP can be seen as a target on the ground whose position is known within a specific coordinate system. This position (of various GCPs) can be used to rotate a drone survey or a laser scan survey into a specific coordinate system, as well as to increase accuracy.

An essential part of using GCPs is that they are distributed “fairly” over the area to be surveyed. If this is not done, the survey can sometimes be distorted according to the location where too many or too few GCPs were placed.

GCPs also increase accuracy because the reference coordinates are known in a coordinate system with an accuracy of 1 to 2 cm (depending on the GNSS device used to measure the GCP). The average drone photo taken with a device without RTK has a positional accuracy of approximately 5 meters. This deviation is corrected in the photogrammetric calculation. GCPs increase the accuracy of these calculations.

The use of GCPs can also ensure that a survey is known in a specific coordinate system, such as the well-known Lambert72 system in Flanders.

However, this could also be achieved with a drone equipped with RTK, which we explain in another blog post.

Nevertheless, GCPs remain the most accurate tool during drone surveys because they provide certainty through a measured position on the ground with a specialized device.

RTK, on the other hand, is measured with a drone by a device that moves and hangs in the air, which also has its advantages and disadvantages.

“Use a GCP that has one clear intersection or center point (with high contrast, for example, a checkerboard tile like this Propeller Aeropoint). However, if you want to use spray paint, spray an L and consistently use the inner or outer corner for measuring and marking in the photos.

In most software such as PIX4D, GCPs are now automatically recognized, so you no longer have to mark them manually in a project! This makes a big difference in terms of profitability for a given project! “.

How many do you need?

Theoretically, at least three (to properly orient, place, and scale the model in all directions). However, at Topo4D, we recommend using at least eight or nine, along with some other markings that can be used as checkpoints. We explain this here. These ensure that we can verify our measurements; the checkpoints are always independent of the calculated 3D model or the 3D point cloud.

What can I use to measure them?

That depends entirely on the desired accuracy. If you want to measure points via RTK (Real-Time Kinematic), this provides an absolute accuracy of up to one to two centimeters. GNSS systems are available for this purpose, including systems from Trimble or the Leica GS18I / GS18T (which we use ourselves). If you occasionally need a high accuracy of two centimeters, a device such as the Emlid Reach is ideal. A complete set is available starting from just over 700 euros. Please feel free to contact us if you have any questions about this.

There are also solutions available for which you do not need a GNSS pole, such as the previously mentioned Propeller Aeropoints. These are Ground Control Points with an integrated GNSS receiver. All you have to do is place the Ground Control Points, turn them on, and leave them for at least forty-five minutes. You can then collect them and send the GNSS data to the cloud. This provides you with accurate coordinates for your points, which you can easily use in Pix4D.

This concludes this brief description of what a GCP is. If you have any further questions, please feel free to contact us, and we will do our best to assist you as quickly as possible.