GIS Files 2: Geographical data

2.7: Topology (3)

Polygon topology

Area features are defined in a GIS by the linear shape of the perimeter and some kind of reference point indicating that the space enclosed by those lines relates to a geographical feature. This reference point is referred to as a centroid, a seed or a label point.

Geospatial data is often captured in the form of linework showing the extent of physical features on the ground, like fences, roads, and rivers. The area features then have to be identified by assigning seed points to each bit of space in the resulting map data. For example, a wall feature may, at one point of its length, define the perimeter of a school playground, but further along form the edge of someone's front garden. It is the seeds that store the information about which links make up the edge of an area feature and what it is. It is very important to avoid undershoots in the data, otherwise the system cannot tell whether an area is closed at a particular point. There will be much more about the structuring of data in chapters 3 and 4.

We have now finished chapter 2: Geographical data - putting the G in GIS. We hope you have found it useful.

Now move onto the next chapter, chapter 3: Adding real-world information - the I is just as important.

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