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Geographic Information Systems

What is a GIS?


Aronoff (1989) described a GIS as any manual or computer-based set of procedures to store and manipulate geographically referenced data. Geographically referenced data (spatial data) are any data that can be represented on a map as a point, line, or area (polygon) (Fig. 1). Storing and manipulating geographically referenced data quickly become too cumbersome by use of manual procedures. Consequently, Aronoff (1989:39) defined a GIS as "a computer-based system that provides the following four sets of capabilities to handle georeferenced data: 1) input; 2) data management (data storage and retrieval); 3) manipulation and analysis; and 4) output."

GIF -- Figure 1
Fig. 1. -- Geographically referenced data (spatial data) are any data that can be represented as a point (A), line (B), or area (C) (Aronoff 1989).

Dueker and Kjerne (1989:8-9) defined a GIS as "a system of hardware, software, data, people, organizations, and institutional arrangements for collecting, storing, analyzing, and disseminating information about areas of the earth." GISs differ from general database management systems (DBMS). DBMS do not handle adequately the spatial data requirements of GISs. Spatial data have two components: a geographic reference, and an attribute. A road in a GIS may be represented by a line. The geographic reference of that line would be the coordinates describing its location. The location of the road (line) can be recorded with Universal Transverse Mercator (UTM) coordinates, state plane coordinates, latitude and longitude, or other (including arbitrary) coordinate systems. The attribute component of the spatial data describing the road may include the type of road (gravel), the route number (Interstate 44), or other attributes of the road, such as the average number of cars that use the road in a year. DBMSs often can manage well the attribute component of spatial data, but poorly manage the geographic reference.

A GIS frequently is described in terms of hardware and software, but it should be thought of as a general system with inputs, processes, outputs, and a context. The input component is the most expensive. Capturing, registering, interpreting, and converting spatial data frequently comprise 60-90% of the expense of operating a GIS. Processes of the GIS include efficient and effective means of storing and retrieving both the attribute and geographic reference of spatial data and creating new information derived from spatial data stored in the system. This new information includes such things as the distance to the nearest stream or the size of continuous blocks of forestland.

The outputs of the GIS include hard-copy maps, graphic displays on color or monochrome monitors, and tabular information. Technological advances are vastly improving output capabilities of GISs.

The context of the system includes the organizational and institutional components of the GIS, e.g., staff, funds, and administrative support. Administering the organizational and institutional components of the GIS is frequently much more difficult than selecting, learning, and using the GIS software and hardware (Lauer et al. 1991).

Many types of systems frequently are confused with GISs (Korte 1991). Computer-aided mapping (CAM) systems automate the design, creation, and maintenance of maps. These systems usually are enhancements to computer-aided drafting (CAD) software and provide powerful tools for making and updating maps. CAM/CAD systems handle well the geographic reference of spatial data, but often poorly handle the attribute components of spatial data. The analytical capabilities of CAM/CAD systems are not the same as those of a GIS. GISs usually do not perform as well as CAM/CAD for purely cartographic application, but the cartographic capabilities of modem GISs are improving. In addition, many GISs now can use and manipulate data created from CAM/CAD systems.

Automated mapping and facilities management (AM/FM) is the use of CAM or GISs for public works and utility information (Ducker and Kjerne 1989, Vonderohe et al. 1991). Information on telephone lines, electrical lines, water lines, sewers, and other utilities often are managed with AM/FM systems. The AM/FM allows linking attribute data to spatial data, but, like CAM, spatial relationships are not defined and spatial analysis is slow and cumbersome at best.

Cadastral systems are used to manage quantity, value, and ownership of real estate. Multipurpose cadastral systems are parcel-based land information systems (Dueker and Kjerne 1989, Vonderohe et al. 1991). These parcels could be sections of the public land survey, counties, or wildlife management units, for example.

Often, the application of GISs is termed Land Information Systems (LISs). Dueker and Kjerne (1989) described LISs and GISs as containing data primarily describing land records. Vonderohe et al. (1991) described the process of maintaining records on the land as an LIS, which does not necessarily require the use of computers. But once this process is automated by computer, it is a GIS.

The five basic questions that a GIS can be used to answer were described by Walker and Miller (1990) as:

(1) What exists at a particular site or location?
(2) Where are certain conditions met?
(3) What changes have occurred over time and where have these changes occurred?
(4) What are the social, economic, or environmental impacts of a particular change in the use of land?
(5) What will happen if the existing land use for a particular site is altered to another type of use?

The first question is one of the simplest functions of a GIS. The location can be described in many ways and can be defined (1) as a point, line, or area (polygon), (2) by place name (i.e., street address, city, county, or wildlife management area) or post or ZIP code, or (3) by geographic coordinates, such as UTM, state plane, or latitude and longitude. A wildlife manager may use this capability to describe the habitat occurring in a wildlife management area, a study area, or a county. A wildlife researcher, using radiotelemetry techniques, may use a GIS to determine various kinds of information about sites used by the species being studied. For each radiotelemetry location, the habitat type occurring at the location can be ascertained. Many other habitat parameters also can be obtained, such as the distance to the nearest road, stream, or forest edge; the size of a continuous block of habitat being used; the elevation, slope, and aspect of the location; or the area of various habitat types located within a determined distance from the location of the studied animal.

The second question that a GIS can answer is the converse of the first question. Instead of asking what occurs at a particular site, this question asks where do certain situations or conditions occur. The wildlife managers may want to know the location of all red-cockaded woodpecker nesting colonies occurring on private lands, which county sells the most duck stamps, or which county has the most area enrolled in the U.S. Department of Agriculture (USDA) Conservation Reserve Programs (CRP).

The third question addresses changes in time. A waterfowl manager may want to know which wetlands are typically dry during the summer and thus have little value for waterfowl brood habitat. A big-game biologist may want to know which counties have had the greatest reduction in forest cover. The GIS uses two or more inventories acquired at different times to address these types of questions.

The fourth question addresses the social, economic, environmental, or combined impact of an existing change in land use. Ascertaining the benefits of CRP to waterfowl populations requires information on the location of the CRP enrollment areas, the land use that previously occurred in the area, the success of establishment of permanent cover on the CRP areas, and the availability of wetlands within and near the CRP sites.

Walker and Miller (1990) believed that the fifth question may be the most important or highest use of a GIS. Using a GIS, a wildlife manager can answer the "What if?" questions. What will be the impact to waterfowl populations if temporary or seasonal wetlands no longer are protected by federal regulations? Which wetlands are most vulnerable to wetland drainage given that drainage rates are a function of distance to nearest road, wetland type, size of continuous wetland, and surrounding land use?

A GIS is not a computerized system for making maps, even though maps are an important output product of GIS and many GISs now have excellent mapping capabilities. A GIS is not a tool for storing maps or pictures (although many modem GISs can store on one CD-ROM disk images of all topographic maps or aerial photography that are found in stacks of map cases or in rolls in obscure corners of offices). Although maps are an essential source of information for a GIS, the information maintained in the database of the GIS is the central concept, not the maps.

A GIS is an approximate model of the real world that uses computer systems to abstract three key pieces of information about features of the land required for management decisions. For every land feature, the GIS must know (1) what it is, (2) where it is, and (3) how it relates to other features (Walker and Miller 1990). GISs provide a mechanism for maintaining information about the land. Gathering information is the first, and most important, step in developing a GIS, followed closely by updating and maintaining information as features of the land change.


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