River databases
The river databases are digital vector databases of the river
areas of the Netherlands. These databases contain XYZ co-
ordinates of the most relevant features, like buildings, dikes
(upper and lower break line) and water lines.
The XY position of the vector data is basis information for the
databases, but it is also used for an easier interpretation.
Opposed to the high way databases, the height information is
very important for the river databases; each hectare contains at
least 4 height points, and a large number of breaklines is
present.
The data is gathered from 1:4000 colour photographs using
analytical photogrammetric instruments.
DEM's for high way construction
For the construction of proposed high ways, DEM's are
generated. These DEM's consist of several zigzagging strings,
making a reliable coverage of the terrain. Next to these strings,
break lines are measured, giving an indication of the most
relevant height changes in the terrain. A high accuracy of Z co-
ordinates is characteristic for this product.
The DEM's are generated with analytical instruments, using
colour 1:3000 photographs.
Coastal base maps
The coastal base maps scale 1:2000 contain mainly contour
lines and vectors of features of the Dutch coastal area along the
North Sea shore. This area is roughly limited from the sea water
line to the fore dunes.
The map contents is gathered from aerial photographs 1:4000
and 1:5000. Although digital techniques are used for the
collection of information, the final product is a hardcopy at
present.
Beach profiles databases
For monitoring the movements of sand masses along the Dutch
sea shore, each 200m a height profile is measured per-
pendicular to and from the sea water line to the fore dunes.
These profiles are measured using analytical instruments with
photographs 1:5000.
Vegetation maps
Vegetation maps present the various vegetation types in river
and coastal areas. Aerial false colour photographs are visually
interpreted with the help of a mirror stereoscope. The
vegetation boundaries are manually delineated on a trans-
parency. In a second stage the transparency's are scanned to
digital images, vectorised by skelettonizing, and visually
corrected.
Road network database
Besides the above mentioned 'high way database,
Rijkswaterstaat (AVV) maintains a database with XY co-
ordinates of all the road axes in the Netherlands, even the
narrow streets in the cities, accompanied with thematic
information. The database is primarily used for accident
monitoring
This database is assembled and updated by combining all kinds
of geographical information; photogrammetry is not being used.
Although this is not a MD product, the MD is to a certain
extend involved in its development.
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2.3 Customers needs
We carry out a continuos research to the customer's wishes in
terms of completeness, accuracy and actuality of our products.
The current Rijkswaterstaat development from isolated manage-
ment for only a few tasks to integral management, results in a
tendency to a need for integral products suitable for these tasks,
for example databases of river areas with height data, directly
linked to databases giving information on vegetation types and
land use.
Thereby the need for highly accurate data decreases, and the
need for actual data increases.
As stated above, the geographical databases are most efficiently
used in a GIS-environment.
Finally, the power of new computers gives us the opportunity to
make visually attractive products, for example bird's-eye-views.
3. DIGITAL PHOTOGRAMMETRY
3.1 The use of digital photogrammetry
For us, digital photogrammetry is defined as a technique for
collecting XYZ co-ordinates of objects using digital images.
The advantages of digital over analytical photogrammetry are
the possibilities to:
1. automatically generate DEM's and display data sets in both
perspective and plan view;
2. produce digital orthophotos, mosaics and bird's-eye-views;
3. implement digital image processing functions (contrast
enhancement, vector on raster overlay, change detection);
4. interface with GIS software for overlay analysis and
modelling applications.
The major drawbacks are data volume, processing speed, costs
and the lower quality of stereo display (Trinder and Donnelly,
1996), (Welch, 1992).
In addition to 'regular' digital photogrammetry with scanned
aerial photographs, we use video images digitised with a frame-
grabber as well. These video images are cheaper in use, but less
accurate. The manner of handling these images for data
collection is principally equal to the use of scanned
photographs.
Another fast, cheap and fairly new technique for DEM
generation is airborne laserscanning. Using this technique
delivers DEM's with a height accuracy and point density
comparative to DEM's generated with digital photogrammetry.
Laserscanning is cheaper and the DEM's are available in an
earlier stage after the flights. On the other hand, digital photo-
grammetry opens the possibility to gather thematic information
like land use, edges of houses, and to generate orthophotos.
3.2 Digital Photogrammetric Equipment
Digital photogrammetric workstations can be subdivided into
three types (Welch, 1992) (Dowman, 1993):
a. high-performance workstations providing a complete range
of photogrammetric tools, like aerotriangulation, DEM-
generation and editing, orthophoto generation mapping, et
cetera. Up to now only a few of these workstations are
available, among these DPW of Leica, Imagestation of
Intergraph and Phodis of Zeiss;
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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