Geographical Information Systems

Cartography, or the art of mapmaking, has been trans-formed in many ways by the use of computers. Traditionally, mapmaking was a tedious process of recording, compiling, and projecting or plotting information about the location, contours, elevation, or other characteristics of natural geo-graphic features or the demographic or political structure of human communities.

Instead of being transcribed from the readings of survey-ing instruments, geographic information can be acquired and digitized by sensors such as cameras aboard orbit-ing satellites. The availability of such extensive, detailed information would overwhelm any manual system of tran-scribing or plotting. Instead, the Geographical Informa-tion System (GIS, first developed in Canada in the 1960s) integrates sensor input with scanning and plotting devices, together with a database management system to compile the geographic information.

The format in which the information is stored is depen-dent on the scope and purpose of the information system. A detailed topographical view, for example, would have physi-cal coordinates of latitude, longitude, and elevation. On the other hand, a demographic map of an urban area might have regions delineated by ZIP code or voting precinct, or by individual address.

Geographic data can be stored as either a raster or a vector representation. A raster system divides the area into a grid and assigns values to each cell in the grid. For exam-ple, each cell might be coded according to its highest point of elevation, the amount of vegetation (ground cover) it has, its population density, or any other factor of interest. The simple grid system makes raster data easy to manipulate, but the data tends to be “coarse” since there is no informa-tion about variations within a cell.

Unlike the arbitrary cells of the raster grid, a vector rep-resentation is based upon the physical coordinates of actual points or boundaries around regions. Vector representation is used when the actual shapes of an entity are important, as with property lines. Vector data is harder to manipulate than raster data because geometric calculations must be made in order to yield information such as the distance between two points.

The power of geographic information systems comes from the ability to integrate data from a variety of sources, whether aerial photography, census records, or even scanned paper maps. Once in digital form, the data can be represented in a variety of ways for various purposes. A sophisticated gis can be queried to determine, for example, how much of a pro-posed development would have a downhill gradient and be below sea level such that flooding might be a problem. These results can in turn be used by simulation programs to deter-mine, for example, whether release of a chemical into the groundwater from a proposed plant site might affect a partic-ular town two miles away. Geographic information systems are thus vital for the management of a variety of complex systems that are distributed over a geographical area, such as water and sewage systems, power transmission grids, and traffic control systems. Other applications include emergency planning (and evacuation routes) and the long-term study of the effects of global warming trends.

From information to navigation

The earliest use of maps was for facilitating navigation. The development of the Global Positioning System (GPS) made it possible for a device to triangulate readings from three of 24 satellites to pinpoint the user’s position on Earth’s surface within a few meters (or even closer in mil-itary applications). The mobile navigation systems that have now become a consumer product essentially use the current physical coordinates to look up information in the onboard geographical information system. Depending on the information stored and the user’s needs, the resulting display can range from a simple depiction of the user’s location on a highway or city street map to the generat-ing of detailed driving directions from the present loca-tion to a desired location. As these systems are fitted with increasingly versatile natural language systems (and per-haps voice-recognition capabilities), the user will be able to ask questions such as “Where’s the nearest gas station?” or even “Where’s the nearest French restaurant rated at least three stars?”