Guide to ground control points for surveying. Learn what they are, how to place them and how they can improve the accuracy of your measurement applications
- A guide to ground control points: what they are and how to place them;
- GCPs are salient points with known ground coordinates. They facilitate high-precision drone mapping;
- GCPs must be visible from the ground and must be evenly distributed over the entire measurement area;
- After placing the GCP, you need to measure its location. Do this with a GNSS receiver that supports RTK or PPK;
- Drones with PPK or RTK reduce the number of ground control points required, but we recommend laying them out as our accuracy reports recommend.
If you plan to do accurate drone mapping, you will need to prepare ground control points for your project.
This article will explain what ground control points are and how to place them.
What is a ground control point?
Ground Control Points (GCPs) are dramatic locations with known coordinates on the ground in your area of interest.
They are used as reference points to establish the exact location of photogrammetric or LiDAR data to the coordinate system you are using.
Ground control points help provide the most accurate aerial map possible. These points are important to the surveyor because their settings are absolutely accurate. This means that a point is related to a real value, such as GPS coordinates.
If you're doing drone surveys and need high-accuracy positioning results, there's no better solution than using GCP.
Even if you are using PPK or RTK on your drone, it is still recommended to use several GCPs to ensure the accuracy of your project.
Terra Drone Europe believes after driving the DJI Phantom 4 RTK that in some cases the accuracy that can be achieved without any GCP is less than 3-4 cm.
For example, the test results show that in the 1:500 aerial survey operation without GCP, the UAV can achieve a horizontal accuracy of 3cm and a vertical accuracy of 5cm, meeting the requirements of aerial survey without GCP.
However, when more accuracy is required, it is recommended to use multiple ground control points (three or four, depending on your site size, as shown in the image above, GCP is the red dot) - but it is still a is reduced compared to the measurement method.
Ground Control Points: Main Requirements
There are many requirements for ground control points.
The main requirement for a ground control point is good visibility so that you can spot it on the photo – as shown in this picture.
Typically, the GCP should be about half a meter by half a meter – 2 feet x 2 feet – and painted in a highly contrasting color to be different from its surroundings. Its center point should also be easily identifiable.
Ground rules for placing GCP for measurement accuracy
The amount of GCP you need depends on the size and topology of your survey site. However, we recommend using at least three GCPs.
It is important that all GCPs are distributed equally within your survey area.
So if you use five points, set one at each corner of the site and the last one in the center of the survey area.
If your work area has significant elevation changes, try placing GCPs at the highest and lowest points of the site.
Make sure the GCP is firmly seated so that it remains stable and does not move easily.
Make sure the GCP is clearly visible from the air and is not covered by vegetation and other obstructions.
After placing the GCP, you need to measure its location.
To do this, use a GNSS receiver that supports RTK or PPK.
Go to your ground control point and center your stick on the target to measure the position of the GCP.
As part of this process, it is important that your pole is level and that you know the height of your pole.
Ground sampling distance
Ground sample distance (or GSD) is another important factor when it comes to drone measurements.
GSD is a computational method used to explain how the real-world scale established by GCP translates into a map.
GSD describes the distance between the center points of two consecutive pixels on a digital image.
So if the GSD of the drone is 5 cm/pixel, it is equivalent to one pixel on your digital map corresponding to 5 cm in reality.
Without an accurate GSD, it would be impossible for surveyors to turn all the drone data they collect into usable maps.
GCP and manual join points
It's worth noting that GCP is different from manual tie points, which are another real-world placement used by surveyors.
Manual connection points are features that can be seen in multiple aerial photos. Surveyors identify these points in aerial mapping software such as DJI Terra, use them to stitch the images together and make a complete photogrammetric map.
While both ground control points and manual tie points are essential in the survey, only the GCP matches the actual coordinate points.
For this reason, manually connecting points can help improve the relative accuracy of the project, but not the absolute accuracy, since their position in space is undefined.
Ground Control Points - Summary
When it comes to measurements, accuracy is king and can make or break a project.
Drones are a great tool for surveying, but precision and accuracy cannot be sacrificed.
GCP helps in this process, and advances in UAS technology mean that less laying is required.
Placing them can be time-consuming, but it's better and faster than redoing the entire survey thwarted by inaccuracy.
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