HUKtOtH
Fig. 3. Part of a river map.
MD takes care of the production of these maps. The MD
practices a method using aerial photographs (panchro
matic and false colourfilm).
Another department of Rijkswaterstaat takes care of the
evaluation of the maps, which are often produced at
regular intervals, in order to study the differences in
vegetation which might be caused by certain projects.
Maintenance of the NAP-network
In The Netherlands some 45000 benchmarks are made
for levelling purposes; the height of these marks is
registered. Reference level is the NAP, the Amsterdam
Ordnance Datum.
The MD is responsible for the maintenance of the bench
marks by checking and updating the heights by means
of precision levelling. Also relevant information regard
ing the benchmarks is published by the MD.
For levelling in the inland canals and rivers as well as in
the coastal area of the North Sea and the Waddenzee the
MD operates a hydrostatic levelling system on distances
up to 12 km by means of a vessel equipped with a leaden
tube filled with distilled water.
Providing geodetic assistance
Geodetic support is rendered for construction and
maintenance purposes, for instance:
- computation and staking out of axis and boundary
points;
- measurement, computation and drafting of cross-
and longitudinal profiles;
- computation of co-ordinates;
- carrying out periodical deformation measurements;
- carrying out special measurements on land and
underwater (for instance during the construction and
positioning of the storm surge barrier-pillars in the
Eastern Schelde Estuary).
Marine Geodesy
Being entrusted with the maintenance of rivers and
NGT GEODESIA 82
coasts and as designer of large scale civil engineering
projects, Rijkswaterstaat has a growing need of positi
oning equipment. The MD evaluates the geometric and
physical qualities of the various systems, keeps abreast
of developments in the instrumental sector and tests the
possibilities of new equipment in practice.
Where positioning is used in conjunction with depth
measurement, there is the problem of combining the
correct depth and location data. The MD has assisted in
the development of a system for automated recording,
which entails the use of electronic desk-top calculators
on board of the sounding vessels.
In the current construction method for tunnels, tunnel
elements are built in a construction dock, floated and
towed over water to the site of the tunnel where they are
sunk. The transport and sinking at the precise location
are supervised by modern positioning systems, the
actual position of the tunnel element being depicted
graphically on a map.
The large scale hydraulic engineering projects carried
out by Rijkswaterstaat necessitate a search for methods
of observing dams, stone revetments etc. under water.
With the help of the MD investigations have been
carried out successfully and a practical method has
been developed for carrying out such inspections under
water with the aid of side-scan sonar.
Maintenance and control of geodetic instruments and
radiolocation systems
A great number of geodetic instruments ranging from
simple levelling instruments to electronic tacheometers
with registration system is in use by the MD and other
directorates within the Rijkswaterstaat. All these appa
ratus are under the administrative control of the MD.
The necessary overhaul and repair of in- and outdoor
intruments takes place in a MD workshop. Furthermore
the MD is in charge of the technical management of the
radiolocations systems owned by the Rijkswater
staat.
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