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158 DE TWEEDE NAUWKEURIGHEIDSWATERPASSING VAN NEDERLAND to be speeded up. Under normal conditions of levelling the instrument's many possible adjustments were never utilized, but they did have an unfavourable effect on the stability of the instrument. The Breithaupt instrument was used in 1923,1925, and 1926, and after fitting with an optical micrometer was brought back for use by the second levelling team from 193 1. 5.1.2 The Hildebrand level In 1927 a new instrument was acquired from the Hildebrand company of Freiberg (Fig. 17). Like the Breithaupt instrument it was fitted with a reversible level ofangle value 5" and a X 40 telescope. It had a more limited range of adjustments, which made the instrument more stable. The tripod had a special upper plate (Fig. 15) which made it uncomfortably heavy to manipulate. In 1930 an optical micrometer was fitted to the instrument. 5.1.3 The Zeiss level: Nivellier A In 1939 a Zeiss instrument was acquired. Unlike the other two instruments, this one was ofthe "dumpy-level" type and was already fitted with an optical micrometer. This instrument was both optically and mechanically more compact and of a more robust construction. 5.2 Staffs 5.2.1 Staffs with Dieperink trapezoidal graduation At the start of the second geodetic levelling (up to 1930) special staffs were used (Fig. 19), designed and described by DIEPERINK [3] [4]. The graduations on the 3 metre long staffs were printed on invar. Practised surveyors can make quick and accurate readings on these staffs; estimates in units ofO.1 mm can be made. Up to 1930 these staffs were erected with the help of special staff tripods (Fig. 29). 5.2.2 The invar staff with half-centimetre line graduation In 1930 the Hildebrand instrument was equipped with an optical micrometer for staff reading. The Breithaupt instrument followed in 1931. For this purpose, new staffs with half-centimetre graduations were employed (Fig. 20). These staffs had two scales, half a centimetre out of line with each other. The readings were made in half-centimetre units; only after totalling by levelling section was the reduction to whole centimetres applied. 5.2.3 Special staffs For reading onto benchmarks a short staff, which could also be used in hanging position, was available. The Dieperink version was a metre long, the version with half-centimetre line gia- duations 1.5 metres. For reading onto the ordnance datum benchmarks in Amsterdam a very special staff was used. 5.3 Miscellaneous Various auxiliary equipment was transported by. the levelling party in a special handcart (Fig. 2 1).
DE TWEEDE NAUWKEURIGHEIDSWATERPASSING VAN NEDERLAND 5.4 Calibration of the staffs Over the years the staffs were calibrated several times following a method described by SCHERMERHORN [23]. The average deviation per metre (Z) after 1923 was at the most 0.1 mm; usually much less. No corrections were applied to the levels to allow for these deviations. Chapter 6 METHOD OF MEASURING 6.1 General In 1926 an "Instruction for first-order levelling" was compiled for surveyors. The measuring method described was subsequently amended in various respects without the instruction being officially revised. 6.2 Setting up of instrument and staffs The distance from the instrument to each of the two staffs was always kept the same. Up to 1929 a maximum value of 70 metres was adhered to for this distance (which is half the back- sight to foresight distance); subsequently 60, sometimes 65, or (in tests) only 50 or 35 metres was used. As far as possible the same staff was always used at benchmarks. There were two surveyors, each making his readings independently for each sight; they adjusted the level setting for each other. 6.3 Use of the level In 1926 only one position of the reversible level was used for measurements; from 1927 both positions were used. With two surveyors, M1 and M2, there are in theory 4 possibilities fora systematic sequence ofreadings, referred to here as methods Al, A2, B1, and B2. During the second geodetic levelling all the methods were employed for a certain period, especially A1 and B2. Each surveyor recorded his readings in his own field book. The instrument was adjusted both in the morning and in the afternoon before the levelling. 6.4 Organization of the levelling Readings were made forwards and backwards, with an interval of a few days in between. A tolerance of 2.5 ,& mm was allowed for the results of the forward and backward measurements. If the tolerance was exceeded, the readings were cancelled and both the forward and backward measurements repeated. Sometimes, for small sections of levelling (R
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