Number of ground survey points depends on the required accuracy
Using existing project survey data as PhotoSat ground reference control saves the time cost of acquiring new survey data. This may include the time and cost of constructing and surveying new ground targets. Advanced projects often have hundreds of thousands of dollars in existing survey data that can be used as PhotoSat ground reference.
Many projects have no reliable GPS and/or Total Station ground survey data. In these situations ground targets can be constructed and surveyed. If this is not feasible, features visible on the satellite photos can be identified and surveyed after the photos have been taken.