A geographic information system, or GIS, is a computerized data management system used to capture, store, manage, retrieve, analyze, and display spatial information. Data captured and used in a GIS commonly are represented on paper or other hard-copy maps. A GIS differs from other graphics systems in several respects. First, data are georeferenced to the coordinates of a particular projection system. This allows precise placement of features on the earth’s surface and maintains the spatial relationships between mapped features. As a result, commonly referenced data can be overlaid to determine relationships between data elements. For example, soils and wetlands for an area can be overlaid and compared to determine the correspondence between hydric soils and wetlands. Similarly, land use data for multiple time periods can be overlaid to determine the nature of changes that may have occurred since the original mapping. This overlay function is the basis of change detection studies across landscapes.
Second, GIS software use relational database management technologies to assign a series of attributes to each spatial feature. Common feature identification keys are used to link the spatial and attribute data between tables. A soil polygon, for example, can be linked to a series of database tables that define its mineral and chemical composition, crop yield, land use suitability, slope, and other characteristics.
Third, GIS provide the capability to combine various data into a composite data layer that may become a base layer in a database. For example, slope, soils, hydrography, demography, wetlands, and land use can be combined to develop a single layer of suitable hazardous waste storage sites. These data, in turn, may be incorporated into the permanent database of a local government and used for regulatory and planning decisions.
GIS software generally allow for two types of data. Some use raster data (i.e., discrete cells in a rigid row by column format), such as satellite imagery or aerial photography, while others use vectors (points, lines and polygons) to represent features on the earth’s surface. Most systems allow for full integration of both types of data. In either case, a fully functioning GIS allows the user to enter or digitize data that are georeferenced; link specific attributes to each feature using relational database management system technology; analyze relationship between various geographic features using a wide range of spatial operations and functions; and produce high-resolution images or graphics on color monitors or plotters.
A GIS can be used to answer basic locational questions such as: What is located at a given point on the earth; or where is a specific feature located? For example, using a mouse-driven cursor, a specific point on a map can be queried to determine its land use, vegetation, soil type, elevation, and land ownership characteristics. Similarly, soils data across an entire watershed can be queried to determine the distribution of areas with hydric soils of greater than 100 acres and are adjacent to a major river system. In the first case, a specific, known point was identified and queried to determine preselected attributes. In the second case, however, specific locations were not known. Rather, the database was searched by the GIS to determine where specific conditions were satisfied (hydric class, size restrictions, and neighboring or adjacent feature characteristics).
One of the more powerful functions of a GIS is that it allows users to synthesize or combine different layers of information to identify resource distribution patterns that may otherwise not be obvious. For example, using various map overlay techniques, threatened and endangered species data may be combined with wetland information to determine if any of the freshwater tidal wetlands in an area provide habitat for sensitive or critical species. This information could be used to develop specialized resource management plans that protect critical wetlands or it could be used to identify areas where the reintroduction of a threatened or endangered species might be successful. This information also can be used in the design of survey strategies and methods to focus on areas of potential threatened and endangered species locations.
A GIS also can be used for complex modeling to answer a wide range of "what if" and ecosystem simulation questions. These may be cartographic models designed to document the co-occurrence or interrelationship of multiple data layers or they may be hypothetical research models designed to mimic natural ecological systems. Similarly, modeling with GIS can be used to predict the impacts that one set of parameters will have on another. For example, wetlands, soils, hydrography, climatology and elevation data can be combined to model flooding within a river system. Upstream changes in land use within the same system can be modeled to determine the potential impact of conversion of a forested floodplain to residential development or to agriculture. As a result, both natural system responses to storm events and the impact of human land use decisions can be assessed prior to the proposed action.
Regardless of the application in which GIS technology is used, these systems provide rapid data access and multidimensional analysis and graphical output capabilities that can result in more effective resource management decisions.
J. Scurry, SCDNR Land, Water and Conservation Division