Ground control is the go-to option for turning drone data into highly accurate, survey-grade models. Visually identifiable Ground Control Points (GCPs) are essential for surveys conducted with drones with a “normal accuracy” GPS, which is accurate to within about 10 meters or 33 feet (the same as a smartphone’s GPS). GCPs are also required in much lower quantities for surveys conducted with PPK/RTK capable drones.
Propeller supports using both AeroPoints and custom GCPs in processing drone surveys. This article details custom GCPs, but you can find more about using AeroPoints here: What Are AeroPoints?
How do I measure my ground control?
The most common method for recording ground control measurements is via an RTK enabled GPS rover using correction data streamed from a nearby reference station. In some cases, ground control can also include known survey marks, which can have a marker placed on top of them to be found in your aerial imagery.
Alternatively, AeroPoints achieve survey-grade accuracy without the time-consuming task of manually measuring and marking chosen locations.
You can also use AeroPoints to create custom permanent GCPs if you don’t have access to an RTK rover. Please see Establishing Permanent Ground Control with Your AeroPoints for more information.
What should my markers look like?
Most markers meet two simple criteria:
1. High-contrast design to be easily distinguished from the surrounding terrain
2. Clear geometry indicating the measured center of the marker
With this in mind, the most commonly used marker designs are either a crosshair or checkerboard design. If you are planning to use ground control but haven’t settled on a design yet, the markers below are a good start.
Please note that your AeroPoints will come with a stencil that can be used to create a clear marker.
Marker distribution depends on what kind of GPS your drone has. To learn more about marker placement and distribution, see Best Practices for Ground Control Placement.
When picking locations to place your ground control, think about what the area would look like from your drone. A GCP is useless if it is obscured by trees, buildings, fences, or power lines in the final imagery. Finally, GCPs should always be placed on the ground and measured to minimize the risk of elevation error in the terrain of the final 3D model. Placing a ground control point on an elevated concrete block (or other elevated structure) can introduce errors in your survey. When marking a permanent ground control location, we recommend that it is flush with the ground surface.
Best practices for capturing your markers
The accuracy of outputs generated by photogrammetry is primarily limited by the ground sample distance (GSD) of the provided imagery, and the same is true for aerial surveys using ground control. If the GSD of aerial imagery is 5 cm and the painted lines defining one of your markers are only 3 cm wide, then there is a fair chance your GCP will be unrecognizable in the final imagery and unable to be used. Making sure you choose a flight altitude to meet a GSD where your markers are clearly defined is crucial; otherwise, they are likely to be impossible to recognize during processing.
*poorly captured markers due to low GSD
As obvious as it might sound, another consideration is to ensure you fly over your markers after marking them. Unfortunately, a common mishap is for pilots and surveyors to plan out ground control only to have the flight path fall short of capturing the marker. If there aren’t enough photos of the GCPs, it will cause processing delays. In some cases, it may be best to plan a flight path that covers an area slightly larger than your intended survey area to ensure you have captured all markers in multiple images.
Best practices for maintaining your GCPs
GCPs will likely weather over time. Depending on the surface, site activity, weather events, and type of GCP, they may degrade or be covered or destroyed quite quickly.
Degraded, covered, or destroyed GCPs will cause processing delays, so it is important to ensure they are well-maintained before flying and uploading a drone survey.
Sometimes it isn’t feasible to check certain GCPs due to access constraints, so here are some tips for ensuring that you get the best accuracy without sacrificing turnaround time:
1. Finding unusable GCPs:
- Check the Processing Report from the previous survey to identify any GCPs excluded by one of Propeller’s GIS specialists.
- If you know of site activity around some of your GCPs, assume they have been damaged or destroyed.
- Consider weather events such as rain.
- Based on site activity and conditions, you will also get a feel for how long it takes before a GCP becomes unusable.
2. Options for what to do with these points:
- Re-mark them and update their location in your GCP spreadsheet.
- Remove them from the GCP spreadsheet.
- Set the GCPs to be excluded when uploading data.
- If you want to verify the area's relative level (RL), and you know it hasn’t changed, you can set the GCPs to be checkpoints in the uploader. What is the Difference Between a Ground Control Point and a Checkpoint in Drone Photogrammetry?
- If you aren’t sure if the GCP is usable, but it is required for accuracy, and you can’t re-mark it, then include it as a normal GCP with the knowledge that Propeller might not be able to use it, and it will cause delayed processing times.
What information do I need to provide to have my survey processed with GCPs?
When you’re finally ready for your GCPs to be processed, we need some information to proceed:
Points in a .CSV file that includes the unique ID of the marker, Easting / Longitude / x: number specifying x coordinate, Northing / Latitude / y: number specifying y coordinate, and Elevation / z: number specifying z coordinate.
Optionally, an additional field called "Purpose" can be assigned. If this column contains the value "CHECKPOINT," then that point will be registered as a checkpoint. Otherwise, it will be included as a ground control point.
EPSG code of the projection in which these coordinates are recorded (for information on the EPSG code for your local region see www.epsg.io).
I still can't do it!
We wrote these articles to arm you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.
If you're stuck, you can connect with our support team by clicking the support button on the top right corner of your user portal.