BACKGROUND IMAGE: iSTOCK/GETTY IMAGES
A GIS (geographic information system) enables you to envision the geographic aspects of a body of data. Basically, it lets you query or analyze a database and receive the results in the form of some kind of map. Since many kinds of data have important geographic aspects, a GIS can have many uses: weather forecasting, sales analysis, population forecasting, and land use planning, to name a few.
In a GIS, geographic information is described explicitly in terms of geographic coordinates (latitude and longitude or some national grid coordinates) or implicitly in terms of a street address, postal code, or forest stand identifier. A geographic information system contains the ability to translate implicit geographic data (such as a street address) into an explicit map location. GIS developers sometimes obtain the map data from public sources or companies that specialize in collecting and organizing geographic information. The process of converting implicit geographic data into explicit or map-form images is called geocoding.
Geographic data can be stored in a vector graphics or a raster graphics format. Using a vector format, two-dimensional data is stored in terms of x and y coordinates. A road or a river can be described as a series of x,y coordinate points. Nonlinear features such as town boundaries can be stored as a closed loop of coordinates. The vector model is good for describing well-delineated features. A raster data format expresses data as a continuously-changing set of grid cells. The raster model is better for portraying subtle changes such as soil type patterns over an area. Most geographic information systems make use of both kinds of data.
GISs do these kinds of things:
- They accept geographic input in the form of scanned-in and digitized map images. Often this data is supplied by a source that may own maps and has already digitized them.
- They rescale or otherwise manipulate geographic data for different purposes.
- They include a database manager, usually a relational database management system (RDBMS).
- They include query and analysis programs so that you can retrieve answers to simple questions such as the distance between two points on a map or more complicated questions that require analysis, such as determining the traffic pattern at a given intersection.
- They provide answers visually, usually as maps or graphs.