definition: Topology describes the spatial relationships between
adjacent features. Polygon features (cadastre, land use) share
edges, line features (street centerlines, rivers) share nodes. Why
do we need topologically correct data? The most important function of a
GIS is the analysis of spatial data. The results of the analysis can
be only as good as the input data.
order to be able to perform network analysis on street or river data,
the data should form a network. This requires connectivity between the
polygon features out of polylines requires connectivity between the
analysis on land use polygons or cadastral polygons require adjacency
(no gaps or overlaps should exist between the polygons.
- The adjacency information is explicitly recorded (ArcInfo coverage).
Line features are represented by Arcs and Nodes. The Nodes have internal
numbers. Each Arc has explicitly recorded "From" and
"To" nodes. Arcs are connected to each other if they share a
common node. Polygon features are represented as set of Arcs recorded
explicitly in the Polygon Arc List. In this fashion there is no
duplication of geometries - the common boundary between two polygons is
represented by a single Arc.
Explicit - Each feature is stored as a single record (Shapefiles,
Geodatabases). Line features (street centerlines) are recorded just as a
set of coordinates (vertices). Node information is not explicitly
recorded. The connectivity between two or more line features
(street intersection) should be enforced by comparing the coordinates.
Polygons are recorder in the same way. Two adjacent polygons are
represented by two records. The common boundary is recorded twice.