With polygon data, one usually starts with many polylines that are combined and cleaned in the first step (Figure 2a-b). This results in fewer polylines with nodes being created at intersections. Then, polygons can be identified (Figure 2c). Sometimes, polylines that should connect to form closed boundaries do not, and must, therefore, be connected either manually or automatically. In a final step, the elementary topology of the polygons can be derived (Figure 2d).