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Title
Proceedings of the Symposium on Global and Environmental Monitoring

651
• A terrain model is useful to indicate plausible road tracks in the image data and
provides constraints related to admissible slopes of lines.
• The hydrography has obvious influences on the appearance of road structures.
Roads often follow countour lines in valleys while minimizing the number of river
crossings.
• An existing roadmap can be used as a guide for image analysis to find analoguous
roads and as a logical framework : new roads are usually connected to existing ones.
We regard a GIS as a digital raster database consisting of layers of cartographic maps
and RS data. It is assumed that the different data layers are registered in such a way
that any two of them can be exactly (pixel-by-pixel) superimposed. To provide a smooth
link between GIS and knowledge-based programming, an object-oriented environment for
image understanding is used, [1]. It is implemented on top of the existing hybrid tool KEE,
see e.g. [5]. Data sets from the GIS can be directly mapped to object classes in a KEE
knowledge base (see Fig. Bl) (in KEE a set of related objects is called a knowledge base
or KB). Possibilities to utilize iconic structures are provided by the image understanding
environment. Expertise about road delineation is added as shown in the next sections.
3 GIS-guided extraction : three case studies
Because of the inherent ambiguity of satellite images and because a variety of road networks
does exist, each showing different scales of complexity, there exists a need for a problem
solving strategy that progressively reduces the search space. To cope with this problem
we start from generic models of road-network appearance and try to incrementally focus
the attention in order to obtain optimal results (with regards to some objective function).
This strategy has been called hierarchical search, [4].
3.1 Exploiting LANDCOVER knowledge
In the first case study, we deal with a generic network model of road appearances that has
the following characteristics :
• the network consists of straight roads
• in general, these roads intersect perpendicularly
• the landcover region is subdivided into repeating geometrical structures (such as
rectangles) by the roads
Typical landcover regions that show this kind of network patterns are forests in flat
terrain (see Fig. L3) or desert areas being cultivated by irrigation projects (see Fig. L5).
On high resolution satellite imagery, these roads are visible as line-like structures. The road
network topology in such areas is related to practical considerations (e.g., accessibility).