13 Building and using topologies

Topological representation recognizes that, in reality, geometric features rarely exist independently of each other. When you gaze down on large metropolises from a plane, you see a maze of streets outlining blocks, interlocked. With a simple geometry model, you could use linestrings to represent the streets and polygons to represent the blocks. But once you lay out the streets, you know right where your blocks will be. Having to create polygons for the blocks is an exercise in redundancy. Congratulations, you’ve discovered topology.

In this chapter, you’ll learn what a topology is, how to build a topology from scratch, and how to use commonly available geometry data. You’ll also learn how to create what are called topogeometries (topogeom) in a topology. You’ll learn how to detect problems in loaded data, fix problems in spatial data, and create simplified geometries using a topology model that maintains the connectedness of the constituent objects.

We’ll be working with two sets of examples in this chapter. The first set will be very simple, created without loading data. This set of examples will give you a feel for how topologies are created and organized. The second set of examples will utilize data from the web. These examples will reflect what you’ll commonly do in your work with topology.

13.1  What topology is

13.2  Using topologies

13.2.1  Installing the topology extension

13.2.2  Creating a topology

13.2.3  The topogeometry type

13.2.4  Recap of using topologies

13.3  Topology of Victoria, BC

13.3.1  Creating the Victoria topology

13.3.2  Adding primitives to a topology

13.3.3  Creating topogeometries

13.4  Fixing topogeometry issues by editing topology primitives

13.4.1  Removing faces by removing edges

13.4.2  Checking for shared faces

13.4.3  Editing topogeometries

13.5  Inserting and editing large data sets

13.6  Simplifying with topology in mind

13.7  Topology validation and summary functions