Horizontal checkpoints shall be established at well-defined points. A
well-defined point represents a feature for which the horizontal position
can be measured to a high degree of accuracy and position with respect
to the geodetic datum. For the purpose of accuracy testing, well-defined
points must be easily visible or identifiable on the ground, on the independent
source of higher accuracy, and on the product itself. For testing
orthoimagery, well-defined points shall not be selected on features
elevated with respect to the elevation model used to rectify the imagery