Observed discrepancy between geodolite and GPS distance measurements

Comparison of contemporaneous measurements of 84 distances in the range of 10 to 50 km by both Global Positioning System (GPS) and Geodolite (an electro-optical distance-measuring instrument) indicates that the Geodolite measurements are systematically longer by 0.283 ?? 0.100 parts per million of the measured distance. Quoted uncertainty is 1 standard deviation. This amounts to 11.3 ?? 4.0 mm at 40 km, which is near the maximum Geodolite range. The systematic difference is within the random uncertainty of an individual GPS-Geodolite comparison and was detected only from an analysis of a large number (84) of such comparisons. The source of the systematic difference has not been identified.