Ground Control Points (GCPs) have been long used in aerial surveys and mapping. GCPs significantly improve the accuracy of orthophotos and drone maps. They help to calibrate the exact coordinates of the drone map with true GPS coordinates. But, what are ground control points and how do you create them? Let’s take an in-depth look into GCPs:
What are Ground Control Points (GCPs)?
Ground control points are physical markings scattered across the survey site. RTK (Real-Time Kinematic) GPS coordinates of these ground control points are measured and recorded prior to placing them across the survey site. GCPs can be as simple as a spray-painted ‘X’ on the ground or a large 2×2 checkerboard. We will discuss in detail below how to make your own GCP.
GCPs are a way of telling the photogrammetry software where the map is located in accordance with the real world. Using GCPs the mapping software matches your map with the actual latitude and longitude of that location (geographic coordinates). This makes your drone map extremely accurate. This is also known as georeferencing.
Where are GCPs used?
Using GCPs can drastically improve the global accuracy of your drone map. However, not all projects require their use. Ground control points are primarily used in projects that exclusively demand precision mapping. Some examples of such projects are construction, agricultural survey, inspections, or topographical surveys. Even a marginal error of a few centimeters can decrease the map’s global accuracy. Therefore, using GCPs may take additional effort but you’re only compromising speed for accuracy. It is always better to get accurate results than to repeat a drone operation.
However, if not implemented correctly, even the use of GCPs can lead to inaccurate maps. Let’s look at how to construct GCPs the right way.
Constructing Your Own GCP
Ground control points can be made in a variety of ways. You can either choose to construct a point with a matte finish and weather-proof materials such as acrylic, plastic, rubber, or vinyl. For reusability or inexpensively spray paint markers for one-time use. It all depends on the nature and requirement of a project. We’ll look at both ways to construct a GCP.
If you’re constructing a GCP with weather-proof materials, make sure that it provides high contrast and is easily identifiable. Using the 2×2 checkboard template is generally useful as it not only provides high contrast but also is easier to accurately mark on the mapping software. Shown below are a few templates that are quite easy to make or acquire and provide high contrast. Additionally, you must make sure that the markers emit little to no glare.
If you wish to create an inexpensive variant, you can spray paint your GCP directly on the survey site. However, the accuracy of spray-painted markers may be difficult to ascertain compared to solid ones. This is because the center of the markers (which represents the geographic coordinates) is more defined on the solid ones as opposed to the spray-painted ones. Thus, while marking your GCPs on the mapping software, there is a slight chance of the coordinates being off by a few centimeters if the painted marker is not defined properly. Given below is an example of a GCP spray-painted with a stencil for better accuracy.
Placement and Measuring the Location of Your GCPs
Before you paint or place your GCPs, it is a good idea to organize their positions. You do not want too many GCPs close to each other. Here are a few ground rules to keep in mind before you place your markers:
- Use a minimum of 4 GCPs, each measuring at least 4 feet for optimum visibility.
- Distribute your GCPs around your site in a way that there shouldn’t be more than one GCP in an image.
- Remember to not place your markers at the edge of the map. Thereby space them 50-100ft from the perimeter.
- Do not place your GCPs in shade, or obstructions. Additionally, if there is any form of topographical elevation on your site make sure to place 1 GCP per major elevation.
As you begin to place your GCPs, it is crucial to measure the GPS coordinates at the geometric center of the marker. This can be achieved using an RTK/PPK GPS receiver. There are plenty of low-cost RTK systems to choose from. However, do not measure the coordinates using a phone or tablet as their accuracy is the same as that of your drone and will not deliver precise results. Alternatively, you can hire a surveyor to measure these coordinates for you.
Setting Ground Control Points on WebODM
Once you’ve measured your GCPs and completed the drone operation over the survey site, the data must be processed in mapping software. While there is a variety of photogrammetry software available to process the data, WebODM cloud is a powerful and inexpensive tool. To learn more about WebODM and how it fares against industry-leading software, read here.
Setting your GCPs on WebODM is a simple process:
- In the GCP Interface panel of WebODM upload the images containing the visible GCPs. Also, upload the measured coordinates of the GCPs in a .txt format.
- After uploading the images and the GCP file, the GPS coordinates will load on the right-hand side. Select each image and match the points corresponding to the GPS coordinates. Once all points have been correctly matched, you can export the final GCP file.
- While uploading all your images in the dashboard to create an orthophoto, upload the final GCP file as well. This will create a GCP overlay on your orthophoto making it globally accurate.
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