utilities and departments of public services and survey
departments.
- Private enterprises, for instance contracting compa
nies for building, utility, road and railway building and
hydraulic constructions, mining companies, indus
tries, project development companies, bureaus con
cerned with architecture and town planning, engineer
ing and consulting bureaus.
The volume of the open market is not known. In our
country no branch has been established with registra
tion of the work put out to contract. Starting from
information known to me, and based on the estimated
total of 700 exployees working with private compa
nies, I estimate the volume of the „open market" at
present to be about D.fl. 50 million.
Currently in this „open market" are working a total of
20 engineering bureaus, surveying and mapping bu
reaus and hydroghraphic surveying departments of
contracting companies and aerial surveying compa
nies. These companies, of which a number present
themselves during this congress, are ready to render
services. They will enable that:
- no extensions of personnel or equipment will be ne
cessary when clients' work increases;
- peak periods will be coped with;
- client can operate in a flexible way with new tempo
rary subsidy measures;
-the simplicity in clients' organization can be main
tained;
- the commercial thinking in clients' organization will be
stimulated;
- in general, a better management will take place;
- greater expertise can be attracted.
In short, they will relieve the jobs of clients and make
possible that the requirements of the latter will be met.
The offer of surveying and mapping activities at
the "Open Market" in the Netherlands
Levelling
From 1976 onwards the Survey Department of the
Ministry of Public Works has systematically put out
to contract considerable amounts of levelling activities
to private bureaus.
Although putting out to contract, photogrammetric
mapping had been the custom for years, this new way of
putting work out to contract was not received with en
thusiasm by the personnel of the Department. The con
tinuous restrictions concerning the staffing of the De
partment however have led to this situation.
It turned out to be the only way in which the Department
was able to continue carrying out its task with regard to
maintaining the system of benchmarks of the NAP-Da-
tum (Amsterdam Ordnance Datum).
Contour survey maps
Also in the framework of the contour survey map of the
Netherlands, the Surveying Department of the Ministry
of Public Works put out work to contract. Yet the map is
almost complete.
Contour surveys that had already been carried out, and
covered 75% of our country, have been combined in this
map. A nice example of what coordination can achieve!
In parts of our country subsidence occurs, due to the
extraction of natural gas.
Setting in peat areas, where the waterlevel has been
lowered, and reclamation of the Lake IJssel Polders also
cause subsidence. In these areas inspection surveys
228
should be carried out, in order to be able to adapt the
contour survey maps if necessary.
Subsidence surveys
In the north of our country an enormously large natural
gas field was discovered in the years 1959-1963, with an
area of about 900 km2 and a producing layer of 70-240 m
thickness, at a depth of 2900 m. The capacity of the gas
field was finally determined to be 2200 billion m3. The
extraction of gas causes subsidence.
To be able to check it, it is, among other things, neces
sary to carry out precise levelling. This levelling has
successively taken place in 1964/65, 1968/69, 1972,
1975, 1978 and 1981, assigned by the Nederlandse
Aardolie Maatschappij and carried out by private bu
reaus, in a motorised way, as well as on foot (figure 1).
The levelling system is formed by 92 loops, consisting of
296 stages, built up from 1862 sections, with a total
levelled length of 1741 km. The number of connected
benchmarks amounted to 1771. The calculation and in
terpretation was supervised to by the Survey Depart
ment of the Ministry of Public Works.
The subsidence in the centre of the area amounted to
about 12 cm already in 1978.
Pipe lines
Because of the discovery of natural gas, which was like
winning the jackpot in the energy lottery, our national
energy supply has been changed drastically. Before
1963 it was mainly provided by coal, coal gas and oil.
Afterwards horticulture, industry, electricity stations
and private consumers have switched to natural gas.
To this end a gigantic system of pipelines was laid at
high speed in the period of 1963-1975. In the years 1978
and 1979 pipelines were made in order to collect North-
sea gas in Northern Holland. The total length of the
completed main pipe line system amounts to about
4000 km.
An interesting fact is, that at the beginning of the sixties
it was generally expected that by the end of this century
cheap nuclear energy would be available. Besides oil
was amply available at a very low, even falling price. So
the exploitation period was limited, for about thirty
years and the Government was worried whether the
Netherlands would ever be able to sell this large supply.
Therefore contracts were drawn up then with foreign
buyers and our main system of pipelines was extended
up to the borders of neighbouring countries. Dutch nat
ural gas is still supplied to Germany, Belgium, France
and Italy, based on contracts from that period.
The laying of the system of pipelines brought about a
great deal of work. So far geodetic survey activities
connected with construction and repair of our infra-
structural facilities, as motorways, canals and railroads
were mainly reserved for government authorities as the
Fig. 1. Motorised levelling in the Groningen Gas Field
NGT GEODESIA 82
