Geographic Information System

Attributes and Features of Physical Space

The information for a GIS database will include the physical elements of a particular space, such as the topography of the area; the locations and names of the rivers, streets, parks, and buildings in a community; social or political boundaries; and attributes of events or incidents, such as crimes or arrests. These map elements, called “features” of a particular space, may be structured as continuous information to be displayed in grids (“image” or “raster” data) or as points, lines, or areas (“vector” data). The relative elevation of terrain in a specific area, as captured through satellite imagery, would be an example of raster, or continuous, data. The exact location, or street address, of a robbery would be shown as a point, an individual street as a line, and a precinct boundary as an area or polygon.

Coordinate System

The display of information on a map requires a framework to allow location to be fixed, or a coordinate system. The most well-known coordinate system is the measurement of the earth by latitude and longitude in decimal degrees, with the point of origin defined by the equator and the prime meridian. The computer software translates systems of coordinates in pairs, with the east–west location (x) followed by the north–south location (y). The process of producing the location of a feature from a coordinate system is called “geo-coding” the feature.

The development of Global Positioning System (GPS) technology now permits a user to record the exact location of an event with a GPS receiver. The 24 satellites employed in the GPS radio navigation system transmit signals to the computer in the receiver so that its current location can be recorded within a few meters. GPS is sometimes used to keep track of criminals sentenced to house arrest or under mandated supervision, such as some sex offenders.

Although the surface of the earth is curved, maps are flat. This problem is accommodated by means of a map projection, a mathematical system that expresses three dimensions as two. In the United States, many map data use the State Plane Coordinate System. The newest mapping software simplifies the use of data based on different coordinate systems in a single map project.

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