The topologic model is often confusing to initial users of GIS. Topology is a mathematical approach that allows us to structure data based on the principles of feature adjacency and feature connectivity. It is in fact the mathematical method used to define spatial relationships. Without a topologic data structure in a vector based GIS most data manipulation and analysis functions would not be practical or feasible.

The most common topological data structure is the arc/node data model. This model contains two basic entities, the arc and the node.

• The arc is a series of points, joined by straight line segments, which start and end at a Arc presents linear features
• The node is an intersection point where two or more arcs meet. Nodes also occur at the end of a dangling arc, e.g. an arc that does not connect to another arc such as a dead end street.
• Isolated nodes, not connected to arcs represent point features.
• A polygon feature is comprised of a closed chain of arcs.

Topology is the “way in which geographical elements are linked together”. Topology is how geographic features are related to one another and where they are in relation to one another.

Topology is the critical element that distinguishes a GIS from a graphics or automated cartography system. It is essential to the ability of a GIS to employ spatial relationships. Topology is what enables a GIS to emulate our human ability to discern and manipulate geographic relationships.

### Labels

A fixed to data points, lines, or polygons.

• Used to describe the feature that you want to map.
• Can include text or numeric descriptors: e. nominal, ordinal, or interval/ratio data types.
• Must be careful in how the different data types are integrated and used – dangerous to mix and match.