Geographic Information Systems

Gis and Its Components

A GIS is like other information systems in that it provides for data input, database management, data manipulation and analysis, and an output format. Yet, it is the way a GIS performs these functions that sets it apart from the other information systems. The essential core of a GIS is that the fundamental units of analysis are geographically based and defined by a coordinate [Page 777]system indicating their north-south, east-west positions. Therefore, the input data and all its attributes (its characteristics and features) are geographically referenced. These data can be features referenced with a single pair of coordinates (points); a series of coordinate pairs corresponding to linear features (lines); or a series of coordinates, with the first and last pairs being the same, forming a boundary around a specific space, thus forming an area (polygon). Thus, the database management component involves working with these different spatial databases.

Manipulating and analyzing the data, because they are geo-referenced, involves using graphic display devices (e.g., monitors) to view the spatial distribution of the data. The manipulation involves many procedures. One such procedure is overlaying different layers of information to form a single map. For example, a city boundary outline (polygon) is overlaid with a street map (lines) and then overlaid with the coordinates of armed robberies (points).

Spatial query and search is another procedure whereby the user can query the database to find and display all the data satisfying a certain criterion or surpassing a particular threshold (e.g., street intersections recording x amount of traffic accidents during a particular time period). A spatial join involves joining layers of information based on a spatial criterion. A point-inpolygon count makes it possible to count the number features in a point layer that are located in a polygon layer (e.g., number of domestic disputes in a particular neighborhood).

Buffering entails drawing a circle of a specific radius around a particular type of feature and then counting the number of a specific type of phenomenon that falls within or outside the buffer. This procedure has become popular for developing maps of the residences of registered sex offenders and assessing their spatial proximities to the activity places of children, such as schools, playgrounds, and day care centers). While there are other GIS data manipulation procedures, the one procedure that made GIS particularly useful for policing was geocoding or address matching.

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