A formula giving the effect of errors in the measured angles and lengths in a geometric figure on the error in an angle or length calculated from the measured values. Often referred to as GDOP. The name refers to the well known fact that the error which results in calculating the value of an angle or length in a geometric figure from the values of other angles and lengths depends very much on the values of those other angles and lengths. There are firm rules set for the shapes allowable in planning a triangulation network, based on this fact.