The principal source of error in a DMI reading is the rigor with which the end points of the observation course are defined. This is particularly true on freeways, where ramps meet travel lanes at small angles, and the point of intersection is poorly defined even on the pavement. Secondly, if the measurement is taken in normal traffic, from a moving vehicle, an operator reflex delay of a fraction of a second can translate to significant distances on the ground. This effect is worst if more than one observer is involved in the measurement. Clearly the worst errors occur at terminal points.
Proportional errors can accumulate over long distances.
Proportional errors are based on exaggerated scenarios Length measurement from GPS is susceptible to other sources of error. GPS inaccuracy caused by selective availability, and output latency caused by the approximately 1 sec computing time within the GPS unit, cause small inaccuracies. With the above instrumental limitations in mind, we measure driven distance along 15 roads, using DMI and differential GPS, and compare these values to the measured road lengths in our 6 databases. There are two reasons for this. Treatment of the cartographic line.