Development of a System Calibration Comparator for Digital Levels in Finland

A high precision digital levelling system uses invar bar code rods and a linear CCD camera technique. The scale of the bar code is a function of temperature. The scale and a constant are determined by the rod calibration. When carrying out digital levelling, the scale of the whole system, in fact the scale given by the instrument, is expected to be equal with the scale of the rod. However, with time, the scale of an instrument and also a rod can change. To check the behavior of the whole system, i.e. rod and instrument together, we have to use the "system calibration" procedure, where the height readings are taken from different sectors on the bar code rod and compared with their true values obtained by a laser interferometer. In the Finnish Geodetic Institute (FGI) automated rod calibrations have been carried out since 1996 using the FGI vertical laser rod comparator and system calibrations since 2002. The FGI system calibration comparator applies elements of the existing FGI rod comparator. Some results of the calibration for the Zeiss DiNi12 systems are given. 1 Importance of system calibration The system calibration (Rueger and Brunner 2000) is used to determine the scale of digital levelling systems, to study their measuring behavior, and to estimate the standard uncertainty of the digital levelling. During the last decades geodetic instruments have become more automatic, converted into measuring systems being externally fi ne constructed and well operating. Software has replaced most of the observer's tasks. Also, the levelling itself has gradually experienced a similar development: The discovery of digital levelling in the beginning of the 1990's led levelling into a new era - levelling became almost totally automated. Previously, when the conventional levelling technique was applied, instruments were simply constructed, but also manufactured with great care applying precision mechanics. Users knew and understood the function of the levelling instruments better and had better possibilities to locate functional faults and even correct small imperfections. Nowadays, it looks like manufacturers have reduced the use of precision mechanics in production, mostly, due to economic reasons. The instruments and the measuring systems are calibrated by the manufacturers themselves and when