Topological query

Standing at a crossroad, holding a situation map in the hands, it is realtively easy to determine which roads are crossing and which buildings are situated next to each other (ESRI). The implementation and further use of such functions in a GIS, however, require some knowledge.

While spatial selection criteria select objects based on their location, and while thematic queries identify elements with regards to their properties, the topological selection criteria are based on the topological arrangement of objects in space. Topological arrangements of objects are accessed through features such as "next", "part of" or "within".

In a GIS, spatial relationships are named "topology". Topological relations are made up of the geometric primitives: Point (simplest element), line (connected points), polygons (connected lines) (ESRI). Based on these structures, the system is able to identify topological relationships and perform analyses.