As indicated in the National Standard for Spatial Data Accuracy (NSSDA):“Horizontal accuracy shall be tested by comparing the planimetric
coordinates of well-defined points in the dataset with coordinates of the same points from an independent source of higher accuracy.Vertical
accuracy shall be tested by comparing the elevations in the dataset with
elevations of the same points as determined from an independent source
of higher accuracy ….A well-defined point represents a feature for
which the horizontal position is known 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 recoverable on the
ground, on the independent source of higher accuracy, and on the product
itself. Graphic contour data and digital hypsographic data may not
contain well-defined points.” In these ASPRS standards, the independent
source of higher accuracy for QA/QC check points should be at least
three times more accurate than the required accuracy of the geospatial
dataset being tested.