In Data checks and repairs we discussed the derivation of elementary polygon topology starting from uncleaned polylines. However, more topological relations may sometimes be needed, as for instance in networks where questions of line connectivity, flow direction and which lines have overpasses and underpasses may need to be addressed. For polygons, questions that may arise involve polygon inclusion: is a polygon inside another one, or is the outer polygon simply around the inner polygon?
In addition to supporting a variety of analytical operations, topology can aid in data editing and in ensuring data quality. For example, adjacent polygons such as parcels have shared edges; they do not overlap, nor do they have gaps. Typically, topology rules are first defined (e.g. “there should be no gaps or overlap between polygons”), after which validation of the rules takes place. The topology errors can be identified automatically, to be followed by manual editing methods to correct the errors.
An alternative to storing topology together with features in the spatial database is to create topology on the fly, i.e. when the computations call for it. The created topology is temporary, only lasting for the duration of the editing session or analysis operation.
Identify the impact of Geo-information handling on data quality (level 1).