What is a GCP?

7 AUGUST 2021

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

GCPs can be sprayed with paint as well as placed through solid targets. Their position is crucial for the quality of the measurement.

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

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

GCPs also increase accuracy as 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 position accuracy of about 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 measurement is known in a certain coordinate system, such as the well-known Lambert72 system in Flanders.

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

However, 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.

Tip from Guillaume, manager of TOPO4D
“Use a GCP that has one clear intersection or midpoint (with a lot of contrast, for example a chessboard tile like this Propeller Aeropoint). However, if you want to use spray paint, spray an L and consistently use the inner or outer corner to measure and mark in the photos.

Nowadays, most software such as PIX4D automatically recognizes GCPs and you no longer have to manually mark this in a project! This makes a big difference in terms of profitability in a particular project! “.

How many do you need?
Theoretically, at least three (to properly orient, position and scale the model in all directions). However, with Topo4D we recommend using at least eight or nine of them, along with a few other identifiers that can be used as checkpoints. We explain this here. These ensure that we can check our measurements, the checkpoints are always independent of the calculated 3D model or the 3D point cloud.

What can I use to measure it in?

That depends solely on the desired accuracy. If you want to measure points via RTK (Real-Time Kinematic) this is done to a maximum of one to two centimeters of absolute accuracy. GNSS systems are available for this, 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 already available from just over 700 euros. Feel free to contact us if you have any questions about this.

There are also solutions available that don’t require a GNSS pole, such as the Propeller Aeropoints mentioned earlier. These are Ground Control Points that contain a GNSS receiver. All you have to do is put down the Ground Control Points, turn them on and leave them there for at least 45 minutes. You can then retrieve them again and have the GNSS data sent to the cloud. This will give you accurate coordinates of your points that you can easily use in Pix4D.

So much for this brief description of what a GCP is, if you have any further questions please do not hesitate to contact us and we will do our best to assist you as soon as possible.