Serious errors can result if the theme lacks sufficient resolution to effectively describe an area of interest. A GIS does not tell its user that she has made an error in choosing the right data layer for her project. 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.