Geographic Information Systems: An Introduction, 3rd Edition

13.4 Organization of Data Storage Operations

Software systems often organize data to ensure effective use. Such organization may involve various logical paradigms concerning the grouping of object types and the divisions of geographical areas. The physical limitations of system file capacities may also be a practical reason for thematic and geographic divisions. A list of all maps in a system, organized by location and theme, forms a map library from which the user can select the map he or she needs and store it in the workspace of the computer.

Thematic layers

Data in most GISs are organized in layers (levels), much like the overlays of conventional mapmaking (Figure 13.2). Similarly, individual data layers are stored in individual data files. This is done to avoid the problem with overlapping polygons, to reduce the actual data volume with which the computer has to work simultaneously, and to structure the data. These layers may, for example, contain object types intended to be processed together, such as points in one layer, lines in another, and polygons in a third. Alternatively, the individual data layers may be organized by theme—perhaps one layer for topography, another for property boundaries, others for roads or types of land use, and so on. Furthermore, each layer may contain subsidiary layers in a hierarchy. Thus, a layer for roads may encompass subsidiary layers for national, county, urban, and private roads. Layers of this type are often called coverages.

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