FAQs: Ground Control Point Rectification

Q. What coordinate system to use for collecting Ground Control Points (GCP)?
Ans: It is best to use the respective UTM coordinates while collecting GCP points. Or the user could also use the respective State plane coordinate system as long as the user is aware of what exact coordinate system was used while data collection and it's EPSG code.

Q. What size/color should my GCP markers be?
Ans: It is advisable that the markers be a distinct color compared to their background. Usually Red or Orange colored markers work best. Other colors that have been used are Blue or Green.
The size of the markers should be big enough that they are visible in images from the flight height like 100 ft -250 ft. It is best if they are 4 ft * 4 ft or bigger.

Q. How many GCP markers do I need?
Ans: It is necessary that we have at least 4 or 5 markers on a project. The markers should be visible in at least 5 images out of the total images or more. If not, then there was not enough overlap in the images and it may result inaccuracies. It is also important to number the corresponding GCPs on the ground, as the AI will detect the number and correlate it to the correct pictures and coordinates. 

Q. Where should the GCP markers be placed?
Ans: The GCP markers should be spread out around the site. E.g.: If the user is flying a bench, there can two markers on the top of the bench, one at each end. And 2 or 3 markers at the bottom of the bench spread out on the floor from end to end.