Topology Topology is a mathematical procedure for explicitly defining spatial relationships . Arc-Node Topology Points along the arc that define its shape are called vertices . Endpoints of arcs are called nodes . Arcs join only at nodes . The GIS also stores the list of arcs that make up the polygon.
Every arc has a direction.
The GIS maintains a list of polygons on the left and right side of each arc. The computer uses this information to determine which features are next to one another. Digitisers A digitiser converts spatial features on a hard copy map into digital format. Point, line and area features are converted into x,y co-ordinates. The process involves manually tracing all features of interest using an electronic stylus.