Geographic Information Systems: An Introduction, 3rd Edition

4.7 Linking Digital Map and Attribute Information

4.7.1 Linking by ID

Common identifiers in map data and attribute data permit moving from map data to attribute data, and vice versa. Attribute data that basically lack georeferencing may be linked to geography. As illustrated in Figure 4.40, this is possible if the attribute data that lack georeferencing have access keys in common with attribute data that have other access keys in common with map data. The connection is then from attribute data to other attribute data to map data.

This illustrates one of the distinctive capabilities of GIS. Data that initially contain no geographical information or referencing may be given geographical dimensions and may therefore be used to enhance and present data in new ways, in maps or on screen. It is not always necessary to link geometry with attributes. In some instances, the geometry can be stored directly together, with the attribute data linked to each register object. This can occur in the case of, for example, a building register with coordinates representing each building or a register of measurement points for use in registering pollution levels. We have also mentioned that identification codes and theme codes are, in principle, attribute data though stored together with geometry.

Figure 4.40

How elements of data in one register can be used as an access key to another register, thereby ...