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DE TWEEDE NAUWKEURlGHEIDSWATERPASSlNG VAN NEDERLAND
Chapter 4 THE REFERENCE DATUM, N.A.P.
4.1 Comparison of benchmarks in Amsterdam
Of the orginal benchmarks dating from 1682, five were still usable in 1875 and two in 1928. The
mean sea level at Amsterdam was measured in relation to the Amsterdam datum during the
period 1700-1861; it is the oldest series ofsea level measurements in the world (Fig. 13). Additional
benchmarks were set up and their levels established by KRAYENHOFF in 1812 and by
STAMKART in 1861.
Information relating to the levels of these benchmarks during the period 1812 (1861) to
1952 has been brought together in table 3 and Fig. 14. It appears from this that the original
1682 benchmarks are the most stable in Amsterdam.
4.2 Comparison of the N.A.P. with the Normal-Null (N.N.)
When links were established with the level network of Germany in the period 1875-1885, the
Normal Null (N.N.) was chosen so that N.N. = N.A.P. From a comparison of the results of the
second geodetic levelling with the new levelling ofGermany it transpires that (average value
along border)
N.N. = N.A.P. - 0.021 m
The standard deviation of the value N.N. - N.A.P. is estimated to have been 13 mm in 1885
and 9 mm in 1940. This gives an estimate for the standard deviation of the change between
1885 and 1940 of 16 mm (see [31]). From this it follows that the 1940 N.A.P. (Amsterdam) has
not changed significantly in relation to the N.N. (Potsdam).
4.3 Linking the second geodetic levelling with N.A.P.
In 1928 the old benchmarks Amsterdam 1 and Amsterdam 3 (from 1682) were included in the
second geodetic levelling (Fig. 15). A special staff was used. The measurements to a number of
other benchmarks in Amsterdam are shown'in tables 4 and 5. The benchmark R.B. Ouder
Amstel3 (N.A.P. + 0.5786) forms the intersection between these levels and the networkofthe
second geodetic levelling.
Chapter 5 INSTRUMENTS
5.1 Levels
5.1.1 The Breithaupt level
The Netherlands Geodetic Commission had at its disposal 4 instruments made by Breithaupt
of Cassel (Fig. 16), specially made for river-crossing measurements [21]. These instruments
had a reversible level with an angle value of 5" and a telescope with a magnification X 40.
It was possible to lift the telescope vertically out of the base with levelling screws (Fig. 16);
in combination with an extra tripod and base with levelling screws this enabled the levelling