What is Topology?

Topology in SIS Desktop 9.1 establishes relationships between geographical elements in a map dataset.

This is particularly helpful if the elements share any geometry. The topological relationship ensures all related elements are edited simultaneously.

The basic building blocks (or primitives) of a topological dataset are Links and Nodes.

  • A Link is a topological LineString item storing relationship data with other topological items.
  • A Node item is a topological Point item used to form the junction of two or more Links (or the end of a Link if it is only partially connected).

Together links and nodes are used to create a network or mesh.

  1. TopoLineStrings represent routes along a sequence of Links
  2. TopoPolygons represent the Polygons enclosed by a sequence of links.
  3. TopoPoints can be created at node locations to represent entities which exist at the node location.

Note: Topology is only supported in Cadcorp Base Dataset (*.bds) or Cadcorp Item Database (*.idb) formats.

Examples of topology

Here are some examples to better illustrate the principle of topology.

A road network is an example of a topological dataset. Nodes represent the road junctions and links represent the road segments between those junctions.

  • The network can be geographically correct depicting every bend and turn in the roads. Or it can be diagrammatic in the same way railway maps are sometimes depicted.
  • The “logic” of the network is inherent in the connectivity of the links, combined with attributes on the links and nodes which set direction of travel and turning restrictions.

A set of administrative or political boundaries is another example of a topological dataset.

In this case it is the polygon formed by the spaces within the network which are important, where adjacent polygons share a common boundary made up of a series of links.

Any change made to the boundary of one polygon will automatically affect the boundary of adjacent polygons as the geometry of the links is shared between those polygons.