The philosophy behind TDN’s decision is that all kinds
of GIS, containing user-specific objects and attributes,
have to be supported. Therefore, data interchange is
crucial and not the data production-environment.
Secondly, TDN itself does not have the intention at this
moment to query or analyse its data. TDN's application
is map production, which does not require a GIS.
The disadvantage of production in a GIS-environment is
its complexity. Not only topological relationships have to
be ensured, also linkages to the attribute data have to be
maintained. Another disadvantage of GIS-production
concerns the costs: GIS-licenses are more expensive than
CAD-licenses. Obviously, the number of attributes to be
stored in a CAD-system is limited. To overcome this
problem, each feature can be assigned to an unique
identifier, which refers to an ASCII-data-table. Moreover,
when the need arises, a GIS-environment can be created
for certain applications.
Although TOPlOvector is produced in a CAD-
environment, a few attributes can be stored, mainly to
describe the features in the database. TOP10vector
contains coded points, lines and areas. Multiple-coding is
applied in order to store the necessary attributes. These
attributes are stored within the applicable geometric
features. The data is structured in a topological way: all
areas are closed and left/right information is added as
attribute data.
One TOP10vector-dataset covers an area of 62.5 km”.
The term dataset is preferred over database and is defined
as the collection of all topographic files covering one map
sheet 1:10,000. A dataset consists of the following files:
BASE-file (BAS):
coded point and line features, topologically structured
(roadsides, ditches), plus centroids, representing the
area code (forest, grassland)
SYMBOLS-file (SYM):
point features together with their cartographic
representation, like post-office, bridge
PATTERNING-file (PAT):
line features together with their cartographic
representation, like row of trees, height differences
BUILDINGS-file (BLD):
closed polygons representing individual buildings
AREA-file (AREA):
closed polygons, computed from the BASE-file.
The AREA-file is fully derived from the BAS-file.
Through the centroïds and their bounding lines, areas are
built which are stored as closed polygons with the area-
codes copied from the centroids. The buildings in the
BLD-file are not topologically linked with the features in
the BAS-file.
A simple example of the content of TOP10vector is given
in figure 2. The centroids of the BAS-file are not
visualized.
The Netherlands Council for Geographic Information
(RAVI) carried out a feasibility-study for a nationwide
uniform geometric reference at scale 1:10,000, a so-called
core database. The study revealed that there is a real need
for such a database. The defined content and structure
corresponds with a selection from TOP10vector-features.
Recently, TDN and RAVI have established a
TOP10vector-user-forum, in which all interested users
can participate. In this forum, user-requirements can be
formulated and, when accepted, implemented in
TOP10vector. Important users are various ministries,
provinces, municipalities, water boards and utility
companies.
2. TOP10VECTOR: A/D-CONVERSION
TDN has a long tradition in regularly updating
topographic maps. In order to keep the map production
going, TDN decided to combine the updating with the
analogue-digital conversion. The production method is
visualized in figure 3.
TOP10vector is based on an intensive reconnaissance
survey, using monochrome aerial photographs at scale
1:18,000. These aerial photographs are enlarged to scale
1:10,000. The process starts with a comparison between
the existing map and the aerial photograph. A positive
film, representing the existing map, is made. Map
features, which are not longer existing, are marked on the
film. New topography is annotated on the photograph.
Not all changed topography can be interpreted. Also
additional information, like land cover classification and
geographic names, has to be collected. Therefore,
topographers carry out a reconnaissance survey after the
preparation in the office. In the field, the same procedure
is followed: non-existing map features are marked on the
NS
BAS BLD SYM
AREA
PAT TOP10vector
Figure 2 TOP10vector
892
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
