A Brief Introduction to Geographic Information System Technology
Enhancing community capacity to use spatial information
Geographic information systems (GIS) link computerized maps (location data) to computerized databases which describe the attributes of particular locations. This technology greatly facilitates the analysis of complex ideas underlying ecosystem management and assessment because a single operator can quickly search, display, analyze, and model a variety of spatial information.
Ecosystem management requires extensive knowledge of the interrelationships of all the biological, physical, and cultural components of the landscape. The geographic characteristics of these relationships are some of the most critical components of this information. The ability of GIS technology to store, describe, and analyze spatial and temporal relationships enhances the ability to assess ecosystem conditions, design management experiments and advance institutional learning.
Geographic information systems are often mistakenly referred to as computerized maps. This perception belies the true nature and power of this information technology. A geographic information system actually links computerized maps to databases that describe the attributes of particular locations on the maps. The power of GIS technology lies in this linkage. The merger of location and attribute data results in a spatial database that has the capability to answer three basic questions: where is it; what is it; and what is next to it?
The where of a spatial database is the specific location represented through a system of coordinates, such as latitude and longitude. The geographic locations of features in a GIS are often referred to as real-world coordinates because they allow us to fit features of the landscape into their proper position anywhere on the earth.
The what refers to descriptions--or attributes--of features in the landscape, such as a stream, a stand of trees or a stream gauge. These descriptions are often quite detailed and may consist of multiple attributes. For example, the attributes of a section of stream may include the name of the stream, stream class, type of fish present, percentage of gravel, temperature, survey date, etc.
The what is next to it of a spatial database refers to the geographic context of features in the landscape. For example, in a vegetation and land cover database an open field may be adjacent to four different vegetation types: a meadow, a late-successional stand, a 10-year old Douglas-fir stand, and a mixed hardwood stand. The knowledge of these complex relationships is inherent to a GIS database and is known as "topology."
Geographic--or spatial--data are expressed as points, lines, or polygons. Water temperature monitoring sites, towns, or waste disposal sites are identified as points. Roads, streams, pipelines and transmission lines appear as lines. Areas sharing a single characteristic, such as administrative districts, forest types, or homogeneous land use types, are represented as polygons. All landscape features can be reduced to one of these three data categories and recorded as a series of latitude and longitude, or x and y, coordinates. These features are linked to data attributes which describe their characteristics.
Data is generally described in terms of map or data layers; each layer representing a particular set of features. Base map data for a GIS, for example, may include separate layers for roads, streams, soils, elevation, vegetation, and inventory plots.
Figure: multiple GIS data layers
Spatial analysis refers to asking questions about spatial relationships using a GIS. Spatial analysis questions may range from the simple (e.g. the number of acres within a watershed), to the complex (e.g. the number of acres of mixed hardwood stands within 300 feet of major streams that intersect publicly owned lands on slopes greater than 30% within sub-basins known to support Fall Chinook salmon). Answers to these types of questions can be derived through the analysis of single and multiple data layers using a number of different analysis techniques such as buffering, overlays and network analysis.
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