GIS Files 2: Geographical data

2.7: Topology (1)

It's all about relationships

While the term topography describes the precise physical location and shape of geographical objects, the term topology is more concerned with the logical relationships between the position of those objects. For example, in a topographic map of Hyde Park, London you would show an accurate depiction of the shape of the park and a precise alignment of the shape of the objects within it – Serpentine lake, for instance.

In a topological map the precise shape of the objects is not important – there will be a shape called Hyde Park and a shape called Serpentine lake, but most importantly the Serpentine lake object will be entirely contained inside the Hyde Park object.

It is the knowledge of this spatial relationship which is key. This may seem a dry and obscure point, but topology is critical to understand how the computer is able to analyse the relationships between objects. If the topology of a set of data is wrong then the GIS cannot analyse how objects relate to each other: are they next to each other? Do they overlap? Do they form a connection? Does one lie completely within another?

Geospatial data will have topology inherited from the source material. Hence, when you digitise a map, the topology, which is implicit in the visual interpretation of the map, is built into the data. However, care is needed. Unless the data is topologically correct the computer will not necessarily pick up the relationships.

One of the commonest errors when digitising data occurs when there is a slight inaccuracy in the start or end point of a line. This can result in the linework not being correctly joined up. The line can form an undershoot or an overshoot (see diagram below).

Although these errors can be difficult to detect by the human eye, they prevent the GIS from understanding the fact that these two features are actually joined to each other.

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