Practical GIS

Vector topology - the right way

More complex geometries have more theoretical possibilities, which leads to added complexity. Defining a point is unequivocal, that is, it has only one coordinate tuple. Multi-points and line strings are neither much more complex--they consist of individual and connected coordinate tuples respectively. Polygons, on the other hand, can contain holes, the holes can contain islands (fills), and theoretically, these structures can be nested infinitesimally. This structure adds a decent complexity for a GIS software. For example, QGIS only supports polygons to the first level--with holes.

The real complexity, however, only comes with topology. Different features in a layer can have relationships with each other. ...