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I. Kabashi, G. Retscher, A. Nießner:
"High Frequent Simultaneous Observation of Zenith Angles using Video Theodolites ";
Poster: Symposium on Geodesy for Geotechnical and Structural Engineering, Berlin; 2002-05-21 - 2002-05-24; in: "Geodesy for Geotechnical and Structural Engineering II", Berlin (2002), ISBN: 3-9501492-1-x; 528 - 530.

Zenith and horizontal angle observations with theodolites are seriously affected due to the influence of turbulent atmospheric conditions, the so-called scintillation and fluctuation, that causes refraction of the line of sight. The influence of the refraction and its variation in time can be obtained using simultaneous reciprocal observations using two theodolites over long observation periods and with high frequency. Observations over long periods and with high frequency are necessary for the investigation of the refraction variation to detect periodical deviations (e.g. daily rhythm and high frequent fluctuations). As this task cannot be achieved using manual observations with conventional instruments, the use of automatic theodolits with integrated CCD cameras (so-called video theodolites) is proposed in our study. At the two stations three essential readings, i.e., the zenith and horizontal angles and their observation time, are taken which are exactly synchronized in time. In previous studies it could be proven that high frequent readings with frequencies up to 25 Hz (i.e., 25 readings per second) can be obtained using video theodolites. The refraction influence can be eliminated if a symmetrical line of sight is assumed.

Field tests were conducted over measurement periods of 24 hours over homogeneous terrain (meadow) for a 125,10 m long baseline. During the observation the weather was mostly cloudy on the afternoon of the first day and mainly fine on morning of the second day. The temperature curve for the observation period is presented. By comparing the variations of the zenith angles with the temperature variations a good correspondence can be seen. This high correspondence was not expected from the beginning, because the zenith angles depend primarily on the temperature gradient, i.e., the variation of the temperature in height, and only secondly on the temperature and the air pressure itself. In this case, the temperature gradients changes equally with time as the temperature changes. However, this correspondence can be seen only until 8:30 a.m.

For the calculation of height differences ΔH the simultaneous reciprocal observations of the two video theodolites were used to eliminate the refraction influence. The difference in height should be a straight line independently of it, when and how long the line is observed. In that case we can say that the refraction is eliminated. The variation of the height differences ΔH show a maximum range of 1.3 mm with a standard deviation of  0.4 mm. As a reference, the height difference ΔH was also measured with precise leveling. Compared to the result of the precise leveling the mean value of the height differences ΔH differs at mm. The simultaneous reciprocal observations of zenith angels with video theodolites show that the developed measurements system obtains good results for high frequent refraction determination over long observation periods.