The use of geographic information systems by local government falls into five major categories:

Browse

This function is equivalent to the human act of reading a map to find particular features or patterns. Browsing usually leads to identification of items of interest and subsequent retrieval and manipulation by manual means. For single maps, or relatively small areas, the human brain is very efficient at browsing. However, as data volumes increase, automated methods are required to effectively extract and use information from the map.

Simple Display

This GIS function is the generation of a map or diagram by computer. Such maps and diagrams are often simple reproduction of the same maps used in a previous manual oriented GIS environment. Examples of this type of use are preparation of a 1:1000-scale town map, a sketch of an approved site plan, maps of census data, etc.

Query And Display

This function supports the posing of specific questions to a geographic database, with the selection criteria usually being geographic in nature. A typical simple query would be: "draw a map of the location of all new residential units built during 1989" A more complex query might be: "draw a map of all areas within the town where actual new residential units built in 1989 exceeds growth predictions." Such a query could be part of a growth management activity within the town.

Queries may be in the form of regular, often asked questions or may be ad hoc, specific purpose questions. The ability to respond to a variety of questions is one of the most useful features of a GIS in its early stages of operation. In the long run, other more sophisticated applications of the GIS may have a higher value or benefit, but to achieve these types of benefits, users must be familiar with the GIS and its capabilities. Such familiarization is achieved through the use of a GIS for the simpler tasks of query and display.

Map Analysis (Map Overlay)

This involves using the analytical capabilities of GIS to define relationships between layers of spatial data. Map analysis is the super-imposition of one map upon another to determine the characteristics of a particular site (e.g., combining a land use map with a map of flood prone areas to show potential residential areas at risk for flooding).

Map analysis (often termed overlay or topological overlay) was one of the first real uses of GIS. Many government organizations, particularly those managing natural resources, have a need to combine data from different maps (vegetation, land use, soils, geology, ground water, etc.). The overlay function was developed to accomplish the super-imposition of maps in a computer. The data are represented as polygons, or areas, in the GIS data base, with each type of data recorded on a separate "layer." The combination of layers is done by calculating the logical intersection of polygons on two or more map layers.

In addition to combining multiple "layers" of polygon-type data, the map overlay function also permits the combination of point data with area data (point-in-polygon). This capability would be very useful in a town for combining street addresses (from the Assessor's files) with other data such as parcel outlines, census tract, environmental areas, etc. Many facility siting problems, location decisions, and land evaluation studies have successfully used this procedure in the past.

Spatial Modeling

This application is the use of spatial models or other numerical analysis methods to calculate a value of interest. The calculation of flow in a sewer system is an example of spatial modeling. Spatial modeling is the most demanding use of a GIS and provides the greatest benefit. Most spatial modeling tasks are very difficult to perform by hand and are not usually done unless a computerized system, such as a GIS, is available. These models allow engineers and planners to evaluate alternate solutions to problems by asking "what if" type questions.

A spatial model can predict the result expected from a decision or set of decisions. The quality of the result is only as good as the model, but the ability to test solutions before decisions have to be made usually provides very useful information to decision makers. Once again, this type of use of a GIS will evolve over time, as the GIS is implemented and used.

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Computer Aided Design (CAD)

A closely related computer capability is a CAD system (computer aided design). CAD systems are used to prepare detailed drawings and plans for engineering and planning applications. While CAD systems functions are different from GIS functions, many commercial CAD products have some of the functionality normally found in a GIS. There are, however, significant differences between a CAD system and a GIS, mainly in the structure of the data base. There may be some need for CAD-type capabilities in a particular local government, so this forms another category of use.

In general, geographic information in local government is used to:

These are typical local government activities which benefit from a geographic information system. The development of GIS will facilitate the present geographic information handling tasks and should lead to the development of additional applications of benefit to the local government.

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