When placing them on the ground you should:

1. Use a minimum of 3 GCP’s per site (as recommended by some photogrammetry software), however I try to use 5 per site, for sites up to 20 acres.

2. 1 additional GCP per each extra 5 acres of the site.

(For example a 100 acre site would need 21GCP's/ 5 GCP's for the first 20 acres + 16 for the remaining 80 acres)

3. Spread evenly over the flight area (one in each corner and one in the centre).

4. Note areas of interest and place GCP’s around them 

5. Placed inside the flight area by at least one flight pass width from the boundary (20Mtrs).

6. Have an un-obstructed view from above.

7. Use extra to allow for any changes in elevation or complex typography.

8. Where the total area is spilt into smaller flight areas (due to size, topography, restrictions etc) then there should be no GCP’s inside of the overlap unless they are present during all flights covering that overlap and unmoved from their original position.