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.