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A Geographic Information System (GIS) enables you to envision the geographic aspects
of a body of data. Basically, it lets you query or analyze 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: population forecasting, and land use planning, to
name a few.
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. GIS
contains the ability to translate implicit
geographic data (such as a street address) into an explicit
map location. 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,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.
GIS accomplishes 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.
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