GCPs: Everything You Need to Know

March 31, 2021

If you work with drone mapping software, you’ve no doubt heard talk of ground control points (GCPs). Used often in the surveying industry, as well as virtual design and construction, GCPs greatly increase the global accuracy of drone maps. Although they are not necessary for every situation, GCPs are a vital tool for precision mapping.

But what exactly are GCPs? And how do you go about using them correctly? To help crack the code on ground control points, we’ve put together this short guide to using GCPs with drone mapping software.

An example of a ground control point (GCP).

What Are GCPs?

In short: Ground control points are large marked targets on the ground, spaced strategically throughout your area of interest. If you use ground control points with your aerial map, you first need to determine the RTK GPS coordinates at the center of each ground target. The ground control points and their coordinates are then used to help drone mapping software accurately position your map in relation to the real world around it.

It might be helpful to think of your GCPs as a series of thumbtacks placed on your drone map. Because the drone mapping software knows each of these thumbtacks’ exact locations, it can reference their locations when it matches up all of the other points on the map.

When to Use GCPs for Your Drone Maps

When used correctly, ground control points significantly improve the global accuracy of your drone map. Basically, they help ensure that the latitude and longitude of any point on your map corresponds accurately with actual GPS coordinates. This is important in situations where precision mapping and true global accuracy are needed, like in surveying or construction - terrain modeling and stockpile management are common use cases here.

Each drone mapping project is unique, and not all projects require a high level of global accuracy. Because of this, it is important to assess each project individually before you decide to take the extra step of using GCPs. But generally speaking, projects like geo-referenced overlays, design documents, and land title surveys benefit from the use of ground control points.

GCPs can look like any of the following.

How to Create GCPs and GCP Checkpoints

There is no one right way to make a ground control point, but an important thing to remember is that the GCP must be easily visible in your aerial imagery. This is achieved by using high-contrast colors and making sure the ground control point is large enough to be seen from your particular flight altitude. We generally recommend flying at 300 feet AGL with a frontlap and sidelap of 70/75 when using ground control points. Keep in mind that this may change dependent upon the area in which you are mapping.

A number of companies do sell pre-made, portable ground control points. However, many drone users simply fashion their own as it is relatively inexpensive and quick to do so. For examples and a step-by-step look at how to create these yourself, review our support documentation.

Similarly, for enhanced accuracy and photogrammetry verification, some users in these target industries may want to create checkpoints for their GCP maps. These checkpoints aren’t used for processing but, instead, validate the relative and absolute accuracy of your GCP maps by comparing the known measured locations of the checkpoints to the coordinates of the same checkpoints shown on your map. To add checkpoints to your GCP process, simply make sure that the name of your GCP label in your .CSV file contains the word “checkpoint.”

How to Utilize Automated GCP Detection with DroneDeploy

Without having to push a button, you’re able to automatically use our Ground Control AI to pinpoint GCP locations on your maps. Instead of manually tagging GCPs in your maps (which you still have the option to do), you’ll instead receive additional messages in your processing emails that detail how many of your markers were tagged automatically, as well as a full accuracy report. This eliminates a host of manual work and speeds you through the map processing section.

Initially trained using over 200,000 human-tagged GCPs, this technology is constantly evolving with input from our customers. By tagging the GCP targets in your images and correcting targets that the assistant misidentifies, you’re training our algorithm to be even smarter and faster. Thanks!

How to Process GCPs & Tips from Our Experts

To ensure your GCPs process accurately and efficiently, it's imperative that DroneDeploy receives specific pieces of information for your project. We’ve created a video overview of how to add GCP information to your maps here for an easy guide to processing GCPs.

Please note that the balance of checkpoints and GCPs you choose will affect how accurate your map is and how quantifiably confident you can be about the accuracy of your map. The more GCPs you use, the more accurate your map is likely to be, but if you have few checkpoints to verify this, it’ll be hard to definitively say how accurate the map is. Using a ratio of 2 GCPs for every 1 checkpoint is a common starting point, with 20 checkpoints being the ASPRS recommendation for getting a 95% confidence interval on the results.

Likewise, we are often asked how many GCPs are required per acre. Interestingly, one seldom-considered factor in this equation is the altitude at which you’re flying. You can think of GCPs as “thumbtacks” and each image as a link in a chain. Having a GCP in an image “tacks” that link to the correct place in space. The more links in the chain between images, the more play there is in the system (the more the chain will be able to move).

Similarly, if we had a GCP in every image, they would all be nailed down. In contrast, if there are many images between two GCPs, there’s more opportunity for sub-pixel errors in the stitching process to accumulate. We understand that you can’t feasibly put a GCP in every image, so we recommend 1 GCP for every 60 images.

If you’re interested in improving accuracy across your flights, download our Making Great Maps eBook. For a 1:1 look at what ground control points combined with drone data can do for your organization, contact us.

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