What is a Ground Control Point?
During an aerial survey, drones collect photos and geographic data. Ground Control Points (GCPs) are physically defined points on the ground, with known coordinates, that are visible in the photos. After a flight, the software processing turns photos and geodata into orthomosaics, 3D models, digital elevation models, or other outputs. Ground control points ensure the accuracy of these outputs by linking the latitude and longitude of a point in a photo to an actual GPS coordinate on the ground.
So what exactly is a Ground Control Point? Anything that is easy to see in an aerial image can be used as a GCP. Commonly, GCPs have a checkerboard pattern or some kind of target with a defined center point. A checkerboard works well because it is high contrast and easy to see in the image. Additionally, the center point provides an exact point that can be measured.
How Do Ground Control Points Improve Drone Survey Accuracy?
Two kinds of accuracy are relevant to data gathered by drones for surveying. To understand how GCPs improve the accuracy of an aerial survey, it is important to understand the difference between relative and absolute accuracy.
Relative Accuracy
Relative accuracy is how accurate points within a project are in relation to each other. For example, imagine an aerial survey with two buildings. If the measurements between the buildings in the images were the same as in the real world, the relative accuracy of the project would be high.
Relative accuracy will usually fall within one to three times the ground sampling distance. (Learn more about ground sample distance.)
Absolute Accuracy
Absolute accuracy is how accurate the items are in relation to their position on Earth. If the locations of the buildings on our survey are not aligned with the base map, they would have a low level of absolute accuracy. If you wanted to use your survey with GIS data, you would need a high level of absolute accuracy, which is where Ground Control Points come in.
Putting It All Together
Depending on your project, it may not be necessary to have a high level of absolute accuracy. If you are performing a volumetric analysis on aggregate stockpiles, you probably only need a high level of relative accuracy. This is because in most cases, the absolute position of each stockpile would not matter. It would be more important to know how much material is in each stockpile.
On the other hand, if you were comparing as-designed versus as-built for a highway project, you would want a high level of absolute accuracy. Adding Ground Control Points to your highway survey would ensure that your images are referencing known geocoordinates. By aligning the Ground Control Points to a base map, you can also ensure that your images are at the correct scale, orientation, and position.
When Should You Use Ground Control Points?
In general, if your project requires a high degree of accuracy or needs to fall within a certain range of accuracy, you would use Ground Control Points. Many engineering and construction projects require 1/10 foot accuracy, so GCPs would be necessary.
Some sources report that GGPs do not further improve accuracy with RTK enabled drones. (Learn more about RTK.) Because RTK corrects the geodata during flight, aligning the geodata to the images during photogrammetry processes would be redundant. When employing RTK systems, Ground Control Points become “checkpoints” that serve to verify the accuracy of a project after it is processed.
How to Use Ground Control Points
Using GCPs is fairly straightforward. Basically, you will lay out points in several areas of your survey, and mark their geo coordinates with a GNSS receiver. Finally, you will upload the Ground Control Point location data into the post-processing software along with the images.
Best Practices
- Make sure the GCP targets are large and easy to see. They should be brightly colored or high contrast.
- Spread your Ground Control Points throughout your survey area. Make sure they are highly visible.
- GCPs should be unobstructed from the air.
- Measure the center of your Ground Control Point with a high precision GNSS receiver.
- GCPs that are numbered will be easier to match up with images in post processing software.
Should I Buy Ground Control Points or Make Them?
Types of Homemade Ground Control Points
- Spray Painted – One of the fastest ways to create a GCP is to use brightly colored spray paint and draw an X or L on the ground. While this is fast and cheap, it is less accurate. Finding the exact center may be unclear, since there is not an exact point to reference. L shapes tend to be easier to pinpoint a corner, whereas X shapes may have some variability.
- Bucket Lids – A brightly colored bucket lid will show up well on images. They are also durable, portable, and inexpensive. You can use electrical tape to create a checkerboard with your center point directly in the middle. Some bucket lids even have an indent right in the middle which will help you align your GNSS receiver!
- Other Materials, Checkerboard Pattern – A checkerboard pattern is more of an industry standard and provides higher accuracy with a defined center point. Depending on size and materials used, it may be a bit more expensive and not as easy to transport. Some materials will not withstand weather or outdoor conditions well.
Where Should Ground Control Points be Placed?
In the most general sense, you will want your GCPs to be spread throughout your survey. For example, if your survey area is a square or a rectangle, place the points at all 4 corners and in the middle. However, it is more likely that your survey area will not be a clean rectangle. With that in mind, here are a few tips that will help you achieve better results:
- Ground Control Points should be on the ground for best results.
- If your survey area has variable elevations, place the GCPs at a low point, a high point, and somewhere in the middle.
- GCPs should be spaced somewhat far apart. Generally they should be placed at each corner and one in the middle.
- There should be some space between your GCPs and the edge of your survey. You do not want to cut one off by accident.
- Know your EPSG code before placing your GCPs. You will need this information for post processing software. WSG84 (EPSG:4326) is a commonly used setting.
How Many GCPS are Needed?
It can be time-consuming to place many GCPs in a large survey. A minimum of four are needed, and usually, four to five are sufficient. Studies have found that more than five GCPs do not improve accuracy. Therefore, the time needed to place ten GGPs would not warrant the negligible improvement in accuracy.
How Do I Measure the Location of the GCP?
In order to measure the center point of each GCP, you will need a high-quality RTK or PPK GNSS receiver. Brands like Trimble and Topcon offer professional quality GPS receivers. Do not use your cell phone or tablet to mark the location as it will not employ the same level of accuracy as one of these professional products.
What are Best Practices for Flights when Using GCPs?
After placing your Ground Control Points are marking the GPS coordinates, it’s time to fly your survey. A few items to consider include the following:
Flight Altitude
For most mapping projects, a flight altitude of 250-350 feet generally provides high enough resolution for many applications. For large projects, you may want to cover a larger area more quickly. In that case, you would fly at a higher altitude. Remember that ground sample distance is affected by altitude. (Learn more about ground sample distance.)
Image Overlap
Post processing software needs some overlap in order to stitch images together. Typically, an overlap of 70-80% is recommended. More overlap improves accuracy, but it does slow down processing times. Additional overlap can compensate for images with edges that are not as sharp as the center.
Using GCPs in Post Processing Software
Depending on which software you employ, the process for using the GCPS will vary. Most software will allow you to import a file or manually enter the GCP coordinates. After entering the GCS coordinates, you’ll need to mark them on the images. For more details, please refer to the software user manual.
Conclusion
Ground control points are the only way to ensure your surveys have a high level of absolute accuracy. They are easy to make or buy, and the workflow is fairly simple. Make sure to use ground control points if your project requires a high degree of accuracy or could be used for legal purposes.
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