GIS explained

A geographic information system is a tool that combines data — such as a hospital's address or a volcano's longitude and latitude — with a visual display.

A GIS doesn't necessarily involve map-making — tables can be generated instead, for example — but building maps is its unique, most popular and perhaps most useful function. After all, said David Nystrom, senior information technology adviser at the Department of Housing and Urban Development, "a picture's worth a thousand words."

"I'm a visualization person," he said. "I just think to be able to see where data is and problems could be and to analyze that is just invaluable."

GIS mapping involves choosing disparate sets of information and laying them over one another onto a base map. According to GIS.com, a Web site run by the Environmental Systems Research Institute Inc., "The power of a GIS over paper maps is your ability to select the information you need to see according to what goal you are trying to achieve."

All of the hardware, software and data involved in producing spatial information in a well-organized way is considered part of a GIS — including desktop or Web-enabling software used for creating, editing and analyzing spatial data.

GIS data is grouped into two types: vector and raster. With vector data, features are mapped according to x, y coordinates, which is good for plotting single points, such as the locations of businesses. Raster data features are displayed as layered cells that gradually change across a map, such as elevations.

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