RIVER NETWORK GENERALISATION BASED ON STRUCTURE AND PATTERN
RECOGNITION
G. Touya
I.G.N., COGIT Laboratory, Paris, France
guillaume.touya@ign.fr
Spatial
database generalization is a key issue in data presentation and integration.
Selection is the generalisation step that consists in choosing the geographical
objects (buildings, roads, rivers) that meet the specifications of the
generalised database. This paper focuses on a specific point, river network
selection. Compared to previous work, this method provides the management of
imperfect and clipped networks, islands and island clusters and irrigation
areas.
Network
selection is a complex process that needs to enrich the initial dataset by
automatically extracting implicit structures and patterns. The main enrichment
used in this work is the creation of "stream strokes" introduced by
Thomson and Brooks in 1999. Strokes are sets of stream segments that follow the
"good continuation" principle of Gestalt theory. In strokes creation,
the key problem is the definition of the continuity rules. In this method, at
each confluence point the main upstream channel and downstream channel are
chosen to continue or end the stroke using the following criteria :
straightness, upstream stroke length or names.
Additional
data enrichments are also used in this method. First, in order to deal with
clipped drainage basins, the database is enriched with river sources and sinks
that are labelled as "natural" or "due to clipping". The
continuity rules for strokes creation are different if the upstream strokes'
source is natural or not. River islands and complex islands (i.e. island
clusters) are also extracted. Furthermore, the stroke creation process based on
flow direction gives incorrect results in irrigation areas. An automatic detection
of such areas allows to use a different process there. Finally, flow direction
on stream segments is recomputed to correct errors present in initial data.
Using all
enrichments, selection step of generalisation can be carried out. Selection
focuses on stream strokes on which Horton ordering is applied. The small, low
order strokes are eliminated. Strokes composing the outline of the islands and
complex islands that are big enough, are kept. In irrigation areas, only
natural streams are kept.
This process
has been implemented in the GIS Clarity™ and tested on the river network of a
French county with heterogeneous landscapes. BD TOPO®, the 1m resolution
database of the French NMA, is used as initial data and the method is
parameterised to derive a 10m resolution database. It provides results
equivalent to BD CARTO® data, the 10m resolution database of the French NMA.