Ground control points are designated locations on a job site that have been precisely measured using Real Time Kinematic (RTK) GPS coordinates. Ground control points, often referred to as GCPs, are used extensively in UAV surveying and construction to increase the accuracy of aerial maps.
In photogrammetry, ground control points are used to georeference the photos taken by a drone or another sensor, ensuring that when the images are stitched together into an orthomosaic, the ground control points are properly aligned. In this vein, more ground control points used generally correlates with higher accuracy. That said, drone mapping software like Site Scan can achieve accuracy within 1/10th of an inch with the use of a minimum of 4 ground control points, spaced appropriately across the survey site.
Ground control points must be visually distinct from their surroundings in order to be easily spotted by manual tagging efforts or automated ground control point detection software. This is often achieved by using high contrast markings and distinct shapes. Often, a spray painted orange or white X, or a black and white checkered pattern are used. Depending on the altitude of a drone survey flight, a ground control point should be of a large enough size to appear clearly in photographs.
Once ground control points are placed, an RTK GPS system, such as a total station, should be used to capture the exact GPS coordinates of the ground control point.
Finally, you can survey your site using a drone, and once uploading of photos has finished, you can begin the process of tagging ground control points in your photographs to ensure accuracy. Drone mapping software like 3DF Zephyr streamlines this process with automated ground control point detection, enabling one to quickly identify the ground control points in your photographs that have been automatically detected by the program.