There is no informational content to the lines except location. Topology establishes the needed information. Topology builds the spatial information by recognizing nodes at the ends of each digitized line and creating new nodes at intersections where lines cross, thereby making true topological chains. They are not functionally connected at this time but are merely two lines standing alone, with no recognized spatial relationship. Image 2shows a river and its smaller tributary.
Topology builds the intersecting node and names the chains.
The river is now two chains, with the new node 4 separating the two features. Because direction is important, the starting and ending node identifications are very useful in defining water flow paths. Image 3 shows a more elaborate version of digitized spaghetti and resulting topological structure for a road system. Study the illustration to understand how topology builds nodes and chains to construct recognizable spatial relationships. Several tables make up a complete topological data structure, as discussed in the next section.