The geolocation of each GCP has to be measured in the EPSG:4326 projection system with ellipsoidal height reference (WGS84).
The surveyor responsible for recording the coordinates of the GCPs should store the original measurement file so that metadata can be retrieved afterwards if necessary, for debugging (e.g. to verify measurements had a fix solution instead of float, standard deviations, …)
The GCP coordinates file to be uploaded to MapEO has to be in .csv or .txt format, following this template: