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160 DE TWEEDE NAUWKEURIGHEIDSWATERPASSING VAN NEDERLAND
the approach lines. A levelling party was made up of two surveyors (initially civil engineers)
and four assistants. The average week's work consisted of more than 32 km of line levelling.
6.5 Records
6.5.1 The level sheet
Fig. 26 shows a copy ofa level sheet with general information at the top about the readings and
the surveyor. In the first column the change points are numbered consecutively in each sec-
tion, and are followed by the distances (foresight and backsight) from the instrument to the
staff and the respective readings. In the column "difference in heightn the difference in fore-
sight and backsight is calculated. The staff used on a benchmark was indicated by means of a
sketch (Fig. 27).
6.5.2 The summary sheet
The results of the levelling were put together on so-called "summary sheets" (Fig. 28). In this
table the calculations for the interim levels of benchmarks, and subsequently the final levels,
were carried out.
In addition, to establish the degree of accuracy, ?/a (= p2LR) was calculated.
6.6 River crossings
Levelling across wide areas of water demanded special measures; see SCHERMERHORN [21].
For this purpose an instrument and a staff were placed on each bank; on these staffs a special
auxiliary staff (with coarse graduations) was attached for reading at a great distance (Fig. 29).
In the second geodetic levelling, four crossings wider than 400 metres were levelled.
Chapter 7 LINKS WITH GERMANY AND BELGIUM
The two German geodetic levellings took place at about the same time as the Dutch ones.
Measurements were taken on five links; the mutual connection of the networks took place
concurrently and with a degree of overlap. The German lines, which together with the Dutch
border lines formed 4 closed loops, were included in the Dutch adjustment, so that a well-knit
connection was formed between the two networks. The geodetic levelling in Belgium was not
carried out concurrently with the Dutch one. At various locations from within The Nether-
lands old Belgian benchmarks, whose levels had been established in 1889-1892, were inter-
sected.
Chapter 8 REDUCTION OF READINGS
8.1 Nodal point correction
From the various check measurements which weIe always carried out at the network intersec-
tions the stability of the benchmark serving as nodal point could be established. A correction