To calibrate the focal length during photogrammetric post-processing and to assess the accuracy of the final materials, before the field work mark the ground targets and survey their horizontal and vertical coordinates, so they can be used as ground control points.
- Prepare in advance either the targets themselves or the materials needed to make them: spray paint, contrasting tape, crosses, nails, etc.
- In urban areas, road markings work very well as targets.
Targets must be placed on a flat surface not covered by any objects (trees, buildings, power lines) - Place at least 10 targets within the flight area. If the site is large, use the formula for the distance between targets: L=R*20 000 pix, where L is the distance between control points,
R is the ground resolution of the images - Marked targets must be clearly identifiable on the photos and on the ground.
- Targets must be distributed evenly over the entire survey area.