Ground Control Points
So what is a GCP? It is any point on the ground, precisely measured with survey-grade accuracy, and whose mark can be clearly identified in drone images.
To use efficient GCP’s on your project you have to:
1. Distribution of GCP’s across the project area
If you have a drone with RTK/PPK positioning you can reduce the number of GCP’s by a factor of 4.
First, place GCPs close to the corners of the project area. Next, locate them evenly across the Area of Interest (AOI).
Avoid clustering GCP’s close to each other – for drones with RTK/PPK you can distribute GCP’s every 1000-1500 feet. For drones without RTK/PPK we recommend GCP’s every 300 feet. You should always place a minimum number of 4 GCP’s regardless of project size. Keep this in mind for small projects. If you go below four you will not be able to measure your quality.
If the area isn’t relatively flat, distribute GCP’s across different elevations – For example, at the top and bottom of a hill.
GCPs should be located on the ground for consistent results
2. Highest possible accuracy of GCP’s
To achieve the highest possible accuracy on your project you should use GCP’s with a checkerboard pattern. This shape leaves very little ambiguity about where to measure your point – always measure at the center of the cross.
You should use GCP’s that have contrasting color patterns. This is why most GCP’s are black and white. It is easier to recognize high contrasting patterns when marking GCP’s on drone images.
3. Measure the GCP’s
Now it’s time to measure the GCPs. In most cases, GCP’s are measured by a GNSS RTK receiver with survey-grade accuracy. If you use a high quality receiver you will save time in the field and this will yield better results.
4. Take flight – use your drone
Fly and obtain high quality images! We fly with RTK/PPK drones like the Wingtra One, but there are many different drones on the market that will produce fantastic results.
5. GCP’s software
Process using GCP’s – leave at least twenty percent of your GCPs out of your calibration, so you can use them as checkpoints.
6. Check the accuracy
Next, we check the accuracy of our processed data by using our checkpoints. Hey, here is a bonus question…. What is the difference between CHECKPOINT and a GCP? They get confused sometimes but the difference is simple. A CHECKPOINT is a designated GCP. CHECKPOINTS look the same and get measured in the same way as a GCP. A GCP actually becomes a CHECKPOINT when you decide it will not take part in the calibration step and will only be used to check the accuracy of your final photogrammetry outputs. In short, any GCP on a project can be used as a CHECKPOINT. . Just make sure you: A) Try not to use the GCP’s you distributed on the corners of your AOI and B) make sure you have the right balance between GCP’s and CHECKPOINTS. We recommend that you have at least 1 checkpoint for every 5 GCP’s.
To check the accuracy of your project, simply compare the location of a CHECKPOINT obtained from direct measurement in the field (GNSS RTK Receiver) and the location of the same CHECKPOINT on a calibrated image or orthomosaic. The difference between these two is your estimated accuracy.
GCP’s are used to precisely georeference a dataset, CHECKPOINTS define the accuracy of this georeference.
SkySnap.com – The right platform for your data
If you are in the drone business, doing a lot of photogrammetry and measurement work like us, you probably have spent some time thinking about the right software to store, share and measure your data. We looked at the available platforms. We knew very well from our experience what kind of challenges drone pilots and businesses who work with both 2D and 3D data face on a daily basis.
We decided to use our experience to create SKYSNAP.COM. Our online software helps drone pilots, construction project teams, engineers/surveyors, asset-managers and anyone who works with 2D/3D data make informed and timely decisions.
Our platform allows you to:
If you are thinking about scaling your services or have challenges with sharing your data effectively with the client, give us a shout at www.skysnap.com. We can set you up with a quick demo so you can FREE your data.
We hope this material was helpful.
We will share the findings on the next key component of quality material namely – quality images in the next piece.
If you have any questions concerning photogrammetry or you want our help processing your drone data, don’t hesitate to give us a shout.
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