AbstractThe necessity and procedures for the regular calibration of EDM instruments have been well recorded (e.g. [2], [11], [12]). However, as the potential accuracy of new instruments increases it is important to ensure that the calibration procedures themselves do not degrade the accuracy of the observations and thus the usefulness of the calibration. This paper discusses this problem in general before giving results of various baselines calibrations carried out using the Geomensor CR 204. This instrument, which is produced by Com-Rad Electronic Equipment Ltd, is a very high-precision lightwave EDM, which has been developed following Com-Rad's involvement with the manufacture of the Mekometer ME 3000. However, the Geomensor has been designed with a completely new and unique principle of operation, which has led to a manufacturer's quoted observational accuracy of±(0.1 + 0.5 ppm). The results of the baseline calibrations described herein show that this accuracy is slightly optimistic for observations under field conditions, where atmospheric effects in particular lead to degradation.