UNTERSUCHUNG DER REFRAKTION BEIM PRÄZISIONSNIVELLEMENT

A most dangerous source of errors of precise levelling consists in the refraction effect of regular character. To reduce the effect of levelling refraction, survey prescriptions of various countries contain different practical measures such as prescription of the mini- mum rod reading, of the maximum admissible rod distance, and specification of daytimes appropriate for precise levelling. These dispositions reduce the refraction effect without eliminating the error due to refraction. In precise levelling, regular refraction effect can be applied for refraction correction, if the temperature vs. altitude function is known. Altitude functions known from the literat ure are rat her awkward by omitting the equilibrium condition of atmosphere in the levelling domain of microclimate, and besides, applicability of some functions is considered only from theoretical aspects e.g. propagation of errors. Thereby, most of the known altitude functions do not agree with the mathematical definition of the meteorological model for microclimate. In this study, the temperature vs. altitude function is deduced by a computerized method of functional approximation. Validity of the function ranges from 10 cm above ground level to the upper level of the unstable lower air cushion. To take the levelling refraction into account, it is feIt to be convenient to replace the temperature vs. altitude function by the determination of temperature gradient from multiannual temperature data by the method of mathematical statistics. The precision can be improved by determining the refraction coefficient as a function of season, daytime, altitude above ground level, and cloud coverage. The present value of the refraction coefficient can be measured by the precise lev- elling of the lapse rate and for that purpose a gradient measuring instrument has been developed