This section describes the best practices for setting, maintaining, and capturing Ground Control Points (GCPs) suitable for creating 3D maps using drones. GCPs are pre-surveyed points used as points of references to establish accurate position of data in 3 dimensional space (x, y, z). GCP coordinate data is used to align and geo-reference photogrammetric data to their correct geospatial position. Visible targets are designed to be easily seen in photos with a center point that can be accurately and precisely marked.
GCP Target Standards
As a best practice, Firmatek standardized on targets with a quadrant pattern with the following characteristics:
- Size: At least 2 ft. x 2 ft.
- Color: Highly contrasting (black and white)
- Center Point: easily identifiable and unambiguous
Securing your GCPs
Large nails or short lengths of rebar can be used to secure targets to the ground. Avoid placing targets where they will be disturbed.
Firmatek also uses AeroPoints, made by Propeller Aero, as a means to set permanent targets or establish GCPs on demand. Characteristics of AeroPoints include:
- They meet Firmatek’s standards, as described above.
- They are a proven & tested product based on proven technology.
- They are easily transported, durable, and have a rechargeable battery.
- They are very easy to use with “one button operation” with WiFi enabled data upload.
- Firmatek processes the GCP data.
For more information refer the Firmatek Using AeroPoints Guide.
Where to Place your GCPs
- Ground Control Points (GCP) should be placed around the perimeter and in the center of the area to be mapped.
- For a small area, a minimum of 5 GCPs/area is recommended; 4 on each corner, 1 in the center.
- For sites >20 acres, there should be 1 GCP/acre placed within 1500 ft of each other.
- GCPs should be clearly visible from the air, free from vegetation and other obstructions.
- Firmatek recommends creating a .kml map of GCPs showing approximate locations of all points.
- Distribute GCPs: For the best results, place your GCPs near each corner of the area to be mapped and distribute more around the perimeter and the center, aiming to cover both high and low elevations. Make sure your GCPs are less than 1,500 feet from the nearest neighboring point.
- Unobstructed: Be sure that your GCP has an unobstructed view from the sky. This means at leas a 15 degree angle in every direction. Avoid nearby buildings, shadows from structures, or tress and bushes that may obstruct clear view of the GCPs.
- Do Not Disturb: Put your GCPs in areas where they will not be disturbed by the operation
- Minimum of 5 GCPs: Smaller areas can use less than 10 GCPs, but there must be a minimum of 5. Below is a good example of a small site with 5 GCPs deployed effectively.
- GCPs require regular maintenance.Regularly check to make sure your GCPs are still in place and visible.
- Avoid moving or disturbing GCPs
- Clear weeds, brush, and dust off the targets prior to a drone flight
GCP Drone Flights
You can find more about flying best practices in other training modules (such as UAV Best Practices, Firmatek Preflight Checklist, and Flight Planning). But here a few things that help ensure your GCPs work well for you.
- Fly mid-morning or mid-afternoon
- Auto-focus should be on by default
- Shutter speed should be set to 1/800
- Windy or gusty conditions can adversely affect the quality of the image and ability to clearly identify the center point of the GCP, so avoid flying in those conditions.
Below is an example of a good (left) and bad (right) quality photo of a GCP.