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| Figure 1: Elements of geographic data. |
This fact sheet provides a general overview of some of the fundamental concepts of geographic information systems (GIS), introducing the reader to specific capabilities and applications. This sheet is the first in a series of fact sheets that will focus on various aspects of the use of GIS in natural resource management.
A geographic information system (GIS) is a computer-based information system that is designed to work with data referenced by spatial or geographic coordinates. Therefore, a GIS is both a database system, with specific capabilities for spatially referenced data, as well as a set of operations for working with these data. Within a GIS, the data are represented as points, lines, or polygons (Figure 1) along with their associated characteristics (attributes) of the phenomena that the points, lines, or polygons represent. For instance, points can represent the location of a heritage or a cultural resource site within a national forest, and the attributes associated with this site may identify the historical period of the American Indian tribe that once inhabited the area. Lines can be used to represent linear features such as roads, rivers, and streams, while polygons can be used to represent area features such as forest stands, vegetation types, or other land uses.
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| Figure 1: Elements of geographic data. |
There are two formats in which geographic data can be represented -- raster-based systems and vector-based systems. The differences between these systems are based in the way in which the earth's surface is perceived, represented, and stored. In raster-based systems, the earth's surface is decomposed into discrete uniform cells, and each cell (pixel) represents a specific area of the earth. Each cell is assigned a value that represents an attribute or characteristic of the geographic location. In such a system, location is inherent in the storage structure, identified by the row and column to which a cell or a pixel is assigned. Raster data are computationally easier to manipulate, but generally require greater amounts of storage space. In vector-based systems, objects as well as their characteristics or attributes are defined. These defined characteristics include the x, y coordinate location. Vector data are often preferred for display purposes, because it maintains a truer representation of an object's shape. Figure 2 provides an illustration of how an object would be represented in a raster and a vector format.
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| Figure 2: Representation of object in raster and vector format. |
A GIS is an integrated system that consists of hardware, software, data, and users. As Figure 3 illustrates, these components interact to address spatial questions that can be used to facilitate more efficient decision-making processes. Overall, the components of a GIS are dynamic. Advancements in technological developments in GIS computer hardware, software, and data acquisition techniques compel users to pursue additional training to develop the necessary technical skills that will support the use of a GIS to its fullest potential.
The United States Geological Survey (USGS) developed a classification scheme that can be used to either identify GIS users as system users or end users. System users are individuals who have worked extensively with GIS hardware and software, and therefore are technically experienced in using GIS as an analysis tool. These users usually have the capa-bility to perform system maintenance and solve application problems. End users are focused on working with the products generated within a GIS framework, and this group of users tends to know enough about the technical capabilities and functions of GIS in order to obtain what they want or need from the system, but their skills do not tend to encompass system maintenance.
Users are the most important component of a GIS. State-of-the-art facilities and comprehensive databases require highly trained individuals. Typically, 80-90 percent of the financial resources of a GIS project are directed to data acquisition and manipulation efforts. Therefore, it is essential for a GIS user to have the necessary skills to process the data as well as have a comprehensive understanding of how the data-base was generated. Computer hardware and software are highly variable components of the system, possessing a wide range of capabilities and costs. Computer sizes are defined by speed, disk space, random access memory, types of input/output devices, number of users, and costs. Purchasing the appropriate hardware and software requires careful consideration of the applications that will be used in a GIS project.
Spatial analysis functions distinguish a GIS from other types of information systems. These functions use the spatial and nonspatial (attribute) data in a GIS database to address questions about the real world. In essence, the GIS database is a model of the real world that can be used to simulate certain aspects of reality. A model can be used to address questions about what exists now, what might exist in the future, as well as what existed in the past. All of these questions involve predictions that occur in time and space. The analysis functions of a GIS provide a user with the appropriate tools to simulate and predict the impact of various management scenarios for a given land area. GIS techniques, such as overlay analysis (Figure 4), buffering (Figure 5), and network analysis (Figure 6) are illustrated here.
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| Figure 3: Components of GIS. |
Since its inception, GIS technology has been strongly associated with the mapping and management of natural resources. Although GIS continues to be used for automating the making of forest maps, attention is largely focused on developing and using the modeling capabilities of GIS software to analyze natural resource issues and concerns. In forestry, some models simply automate the calculation of timber yields, help select timber for either harvest or conservation, or analyze potential forest-management alternatives. Within this decade, GIS will be used increasingly to interface with predictive models, providing forest managers with a critical tool to aid in the analysis and comparison of integrated resource- management alternatives.
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| Figure 4: Overlays. |
GIS-based modeling approaches have emerged to integrate wildlife habitat concerns in the forest management process. Spatial data can be used to determine the availability of viable habitat for specific species types as well as estimate the habitat needs of a given species by observing wildlife populations at specific locations. Regardless of the application, GIS is proving to be an effective mechanism to manage the diverse interests, resources, and demands that are encompassed by natural-resource development. Developed and applied correctly, a GIS can provide resource managers with powerful information that will aid them in the development of resource-management and planning decisions.
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| Figure 5: Buffers. | Figure 6: Network analysis. |
Burrough, P.A. 1986. Principles of geographical information systems for land resources assessment. Oxford Science Publications.
Demers, Michael, 1997. Fundamentals of geographic information systems. John Wiley & Sons.
Nichols, Joan, Rose Dietmar, and Robert Haight. 1997. Prediction of favorable pine marten habitat on the Superior National Forest. FORS Compiler 15(1):19-27.
Parker, H.D., ed. 1989. The GIS source book. GIS World, Inc., Fort Collins, CO.
USGS. 1988. A process for evaluating geographic information systems. U.S. Geological Survey open file report 88-105. 22p.
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