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156 DE TWEEDE NAUWKEURIGHEIDSWATERPASSING VAN NEDERLAND
Chapter 2 BACKGROUND AND PROGRESS OF THE SECOND GEODETIC
LEVELLING, 1926-1940
2.1 Background and commencement of levelling
Measurements made between 1914 and 1920 repeatedly showed that the results of the first
geodetic levelling had become out of date. The Netherlands Geodetic Commission advised
the Minister for Public Works that a revision of the geodetic levelling was required.
In 1926 the Geodetic Bureau (Prof. W. SCHERMERHORN) was commissioned to do this work
by the Department. In 193 1, the Geodetic Bureau was closed down. The staff were transferred
to the Department for Public Works and formed a new section, the Survey Department (Meetkundige
Dienst). In the period 1931-40 the Survey Department completed the levelling.
2.2 Progress of the field work
The lines levelled over these years are summarized in this paragraph.
2.3 Staff involved in the second geodetic levelling
Names of members of staff.
Chapter 3 BENCHMARKS
3.1 Normal benchmarks
In the second geodetic levelling, exclusive use was made of round bolts (20 mm diameter)
bearing the inscription N.A.P. Various alternative types are to be found which were employed
in previous levellings.
3.2 Underground benchmarks
In order to establish durable reference points for N.A.P., underground benchmarks were pro-
vided throughout The Netherlands. An underground benchmark consists of a bronze bolt sunk
into a concrete or granite pillar about a metre high, placed in stable Pleistocene deposits. First
order underground benchmarks consist of a group of 4 or 5 of such pillars spaced at distances
of 11 to 40 metres from each other.
Usually one of the pillars protrudes above the ground (the visible mark, "B.M.") (table 1).
Second order underground benchmarks consist of only one pillar (table 2).