254
Due to the reclaiming of the Zuyderzee it was possible to project a new
central point, Lelystad, and to measure this point in the hexagon Enkhuizen-
Urk-Kampen-Harderwijk-Naarden-Monnikendam, by which a little omission
in the original primary network has been abolished at the same time.
There was a gap in the connection of the Dutch primary network in the
north-east of the country with the adjoining German network. In co-operation
with the "Niedersachsisches Landesvermessungsamt" a supplementary
network was projected between Finsterwolde and Bentheim which was
measured in 1957. More southwards the connection with the primary network
of Nordrhein-Westfalen was insufficient. In the pentagon Hettenheuvel-
Flierenberg-Venray-Hinsbeck-Fiirstenberg, with Kevelaer as central point,
the directions in this central point were lacking. These were measured in co
operation with the "Landesvermessungsamt Nordrhein-Westfalen" in 1957.
All measurements were performed by daylight with the theodolite Wild
T 3, with as bearing points Francis- or Bosch-searchlights with a light-
intensity of some 200.000 or 300.000 candle-power. At present experiments
are made with a new Philips mercury valve, which, mounted in a Francis
searchlight, has a light-intensity of some 5 or 6 million candle-power.
At the beginning of 1957 preparations were made for the measurement of
a standard base in the Loenermark in order to dispose of an internationally
uniform scale, for measurements in the own network as well as for measure
ments in north-west Europe. Pillars were built at o, 1, 6, 24, 96, 288 and 576
m, the first two of which are combined. The underground as well as the
aboveground bolts were placed by Dr. Kukkamaki and Dr. Honkasalo who
finally determined the distances according to the light-interference method
by Vaisala with the aid of the interference comparator (10).
On June 1, 1957 a Dutch geodetic-astronomic station was installed at the
Isle of Curasao, West-Indies. On the occasion of the International Geophys
ical Year 1957-1958 two geodetic engineers will make observations there
for a year and a half (11) and (12).
At Kimaam, Erambo and Merauke in Netherlands New-Guinea the Hydro-
graphic Office Royal Netherlands Navy have taken astronomic observations
for the determination of latitude and longitude. The method Roelofs, applied
here, is described in (13). In collaboration with the Royal Netherlands Navy
under the auspices of the Netherlands Geodetic Commission (Rijkscommissie
voor Geodesie) about 180 gravity measurements extending across the whole
North Sea have been made, while the observations west of Panama, being
made in the same way, are still continuing. Between De Bilt, the national
reference station, and Eindhoven a calibration base-line has been established.
This base-line is measured with three gravimeters which for this purpose
were calibrated on the German base-line Harzburg-Torfhaus first. The
difference of gravity between De Bilt and Eindhoven is 86 mgal.
On the occasion of the seventieth birthday of Prof. dr. ir. F. A. Vening
Meinesz a memorial book has been edited in which a large number of authors
pay tribute to this great geophysician (14).
Little change has come about in the method used in primary leveling. An
effort was made to measure the difference in height only one time per stroke
but later on this method was abandoned. At first only one instrument type
Zeiss-A for primary leveling was available. In the course of the period under
review two instruments, Wild N III, with leveling rods with centimetre
divisions and two instruments, Fennel-Plani, with leveling rods with half
centimetre divisions were purchased. Both types of instruments give about
the same accuracy.
In measuring river-crossings a hydrostatic leveling was used once with the
help of a tube of 4200 m (15). River-crossings were also measured with the
instruments Wild N III and Zeiss-A in which the correction for being out of
adjustment was determined and taken into account (16).
For the second-order and third-order leveling the automatic instrument
Zeiss Ni 2 was increasingly used. For second-order leveling the optical
micrometer was employed.
