DroneDeploy

Capturing Ground Control Points (GCPs)

How to Successfully Use Ground Control Points for Drone Mapping

Check out our 6-minute overview video for a summary of this article

Ground Control Processing Access

GCPs are available to Enterprise Level Customers and are also available to Business users for $49 per map. If you are currently on a Pro or Business plan and are interested in GCPs, please contact support@dronedeploy.com. We can help determine if GCPs are the best option for your mapping project.

Ground Control Points are an effective way to increase the absolute accuracy of a map (more about mapping accuracy here).

GCPs must follow the GCP Checklist or they will not be able to be processed.

Please Note, GCPs maps that meet these additional conditions will process in the fastest time possible!

  • GCP locations are measured in WGS84 (EPSG Code: 4326)
  • Requests containing 10 GCPs or less
  • Requests containing 1000 images or less
  • GCPs are unobstructed and clearly marked with an X or checkerboard larger than 4 sqft.

Need a Refresher on Projections, Reference Systems or Sea Level?

Check out this guide on the many intricacies related to global accuracy to help better understand and improve upon your map's accuracy.

Now let's go through how to capture GCPs for drone mapping.

1. Create at Least 4 Quality GCPs on the Ground

The first step in processing GCPs is to create clear GCP markers evenly distributed around the area you are mapping.

An ideal GCP marker can be as simple as two intersecting lines. The goal is to create a visible feature on the area that is being flown. The GCP should be visible on the map at the given altitude that the mission was performed. Remember, if you cannot clearly see the center point of the GCP in your map photos, it cannot be accurately marked during processing. If your GCP is obscured, poorly marked, or if you cannot confidently place your cursor on your photos within an inch of the point where the GCP was measured with GPS, it does not matter that your GPS took a centimeter or millimeter accurate reading, your GCP will only be as accurate as the center can be marked.

Adding a center mark to the GCP, as seen in the images below, can also eliminate confusion as to where the center point is located.

Example of a well constructed GCP. The marker is large enough that it is visible from far away, but the location will be measured from the center to increase accuracy.

Example of a well constructed GCP. The marker is large enough that it is visible from far away, but the location will be measured from the center to increase accuracy.

If you cannot mark your GCPs with paint there are a variety of low-cost ways to make excellent markers with items from the internet or home depot. These sheets of roofing rubber ($4.66 each) with these vinyl stickers ($1 per 6" x 12" sheet) can be used to make a simple checker pattern but they need to be staked into the ground if its windy.

These rubber and vinyl markers end up costing $5 each and are extremely weather resistant.

These rubber and vinyl markers end up costing $5 each and are extremely weather resistant.

A fantastic DIY approach we have seen is a 12" bucket lid stuck on a white plastic 14x14 access panel from Home Depot

A fantastic DIY approach we have seen is a 12" bucket lid stuck on a white plastic 14x14 access panel from Home Depot

A less weatherproof but disposable idea is marking cardboard cake circles (shown below) with a sharpie and securing them with a 6" magnetic nail for easy recovery.

Placement of GCP points should be scattered throughout the area of interest being mapped. If the area being mapped has noticeable elevation changes, make sure your GCP placement (hills, mines, valleys etc.) accounts for this by placing them in the different elevations on a relatively flat surface. Ensure that the region of interest (where you want the map accuracy to be highest) has GCP points surrounding it, as well as scattered throughout the region. When placing GCPs around the perimeter DroneDeploy recommends a 50-foot buffer zone between the edge of the map and the GCPs location. This will ensure that there is enough image coverage to carry out the reprocessing.

Measuring GPS from the center of the point helps increase accuracy.

Measuring GPS from the center of the point helps increase accuracy.

Tips:

-Take the coordinate from the center of the intersecting lines' GPS coordinates.
-Use an RTK or PPK GPS system for the most accurate reading.
-Fly with a high-resolution (20MP or greater), mechanical shutter camera like the DJI Phantom 4 Pro.

How many GCPs and where to place them?

DroneDeploy requires a minimum of 4 GCPs. No more than 10 are usually needed for larger maps. Please note that if you decide to use more than 10 GCPs, the processing time for your map will increase. We recommend 5 GCPs located in the 4 corners of your map and the center as shown below. Again, make sure your GCPs are at least 50 ft from the boundary of your map.

In this example, the subjects of interest are the 2 star shaped buildings so we place our GCP as best we can in the corners and center.

In this example, the subjects of interest are the 2 star shaped buildings so we place our GCP as best we can in the corners and center.

In the example the subject of interest is not a square so we distribute the points as best we can in the corners and center. You can also see some other measurement annotations.

In the example the subject of interest is not a square so we distribute the points as best we can in the corners and center. You can also see some other measurement annotations.

2. Conduct Your Flight

GCP flights are flown just like any other flight. The only difference is that you are now capturing GCP targets in your imagery that will later be used to increase the accuracy of your map. Make sure that your camera is in focus and setup correctly. If the GCP targets are not in focus the process will not result in the same levels of accuracy. It is always a good idea to make sure the area you will be flying that day has good weather. High-winds, low-clouds or precipitation can make flying and mapping difficult. For more information, please see this article: Making Successful Maps.

We highly recommend flying with a high frontlap and sidelap (80/80) and below 200ft if you a looking maximize the accuracy of your GCP map. We also recommend flying with a high-resolution (20MP or greater), mechanical shutter camera like the DJI Phantom 4 Pro. With a Phantom 4 Pro flying below 150ft you can capture images with a ground sampling distance of ~0.5 inches per pixel. By also including RTK GCPs you can achieve the best ASPRS RMSE's ranging from 2cm-5cm. This is then ASPRS certifiable in horizontal classes of 2.5cm-7cm. See the full ASPRS accuracy standards here.

3. Upload Your Images to DroneDeploy

Follow typical protocols in Uploading Images to DroneDeploy.

4. In-Browser GCP Tagging with DroneDeploy

Check out our walkthrough of the in-browser GCP tagging workflow for details on how GCP's are processed with DroneDeploy.

Capturing Ground Control Points (GCPs)

How to Successfully Use Ground Control Points for Drone Mapping