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DE TWEEDE NAUWKEURlGHEIDSWATERPASSING VAN NEDERLAND The remarkable constant parts of the formulas for g, found in practice exhibit some relation- ship to the weight of the instrument plus the tripod used and could therefore indicate settling of the instrument. Furthermore, an investigation of the optical micrometers brought to light the fact that in the case of the Hildebrand instrument there was a periodic error present, and in the case of the Breithaupt instrument a scale error (table 25). The major part of these effects will certainly have been eliminated by the averaging of the two surveyors' results. 10.3.3 The standard deviation per kilometre of levelling calculated by different methods Extrapolation of the accuracy of reading per sight derived in the laboratory leads to a theoreti- cal standard deviation U, of 0.2 mm per kilometre with a sight length of 2 X 60 m. This refers solely to the instrument's contribution. The standard deviation per kilometre U, (table 26) which was calculated using the standard deviation per sight g, found in par. 10.3.2, also proved to be significantly less than the value UR = 0.66 calculated in par. 10.1 (see also table 14). For comparison, a standard deviation uk per kilometre levelling was calculated from the difference between the average back and forth levels found by each of the surveyors: instead of from the difference of the average of the two surveyors' back and forth levels: From table 27 it appears that uk is always < u~, which indicates a high correlation between each surveyor's levelling (carried out simultaneously). The values uk are comparable with U,. 10.4 Comparison of the results of the first and second geodetic levelling Of the l l00 benchmarks from the period 1875-1885 and 1886-1887 approximately 500 were still in existence at the time of the second geodetic levelling. The changes in levels are recorded in table 28 and shown in Fig. 38. There are especially drastic ones among the cases of settlement. EDELMAN [34] selected the most reliable of these data and drew up a crustal movement chart (Fig. 39) taking into account geological interpretations (PANNEKOEK). At the same time he drew attention to the high degree of guesswork involved in producing the chart. The standard deviation of the observed changes along the periphery of the network is estimated at approx. 12 mm; in reality, however, this figure may be higher. Moreover there is a major discrepancy between the results of the Dutch and German levellings, especially in the northern part of the border area (Nieuwe Schans). In order to strengthen the first geodetic levelling network a new adjustment was calculated in 1965, in which the 1886 and 1887 data [l61 and the misclosures of the Dutch-German border loops (KNEISSL, [12]) were utilized. The most important differences from the original adjustment are shown in table 29. The significance of a number of changes in eastern Groningen has been removed as a result
DE TWEEDE NAUWKEURIGHEIDSWATERPASSING VAN NEDERLAND 165 of this new adjustment (table 30). Nevertheless the general impression from Figs. 38 and 39, of a tilting about an axis running roughly East-West, is retained. Chapter 11 OBSERVATIONS AND RESULTS In table 32 all the measurements together with weighting coefficients, corrections, and calculated levels, have been tabulated for each loop. For the sake of completeness the currently valid benchmark number has been included in the table. The survey date is indicated by a code as follows: Side lines to important benchmarks which do not form part of a continuous line are included per loop underneath the summing up, labelled with the relevant line number accompanied by the letter a, b, etc. (for example 4a). The benchmark mutual to both lines is indicated by an asterisk. For mathematical reasons the calculation of the definitive levels has been carried out in 10-S metres; it is obvious that the last digit does not have any practical significance. Where appro- priate, a reference is given in the last column to the other loop ofwhich the line or benchmark forms a part.
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PUBLICATION OF THE NETHERLANDS GEOD
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