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GEOGRAPHIC INFORMATION SERVICES (GIS)
How GIS works
A GIS stores information about the world as a collection of thematic layers
that can be linked together by geography. This simple but extremely powerful and
versatile concept has proven to be invaluable for solving many problems from
tracking delivery vehicles, to recording details of planning applications, to
modeling how a brush fire will move across a landscape.
Geographic information contains either an explicit geographic reference such
as a latitude and longitude or national grid coordinate, or an implicit
reference such as an address, postal code, census tract name, forest stand
identifier, or road name. Implicit references can be derived from explicit
references using an automated process called "geocoding." These geographic
references allow you to locate features (like a business or forest stand) and
events (like a chemical spill) on the surface of the earth for analysis.
Geographic information systems work with two fundamentally different types of
geographic information--the "raster model" and the "vector
model."
In the vector model, information about points, lines, and polygons is encoded
and stored as a collection of x,y coordinates. The location of a point
feature, such as a bore hole, can be described by a single x,y
coordinate. Linear features, such as roads and rivers, can be stored as a
collection of point coordinates. Polygonal features, such as sales territories
and river catchments, can be stored as a closed loop of coordinates. The vector
model is extremely useful for describing discrete features, but less useful for
describing continuously varying features such as soil type or surface elevation
data.
The raster model evolved to model such continuous features. A raster image
comprises a collection of grid cells rather like a scanned map or picture. Every
cell can have a unique value. Both the vector and raster models for storing
geographic data have unique advantages and disadvantages and modern GISs are
able to use both in concert to effectively perform the most complex analysis
tasks.
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RESOURCES
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CONTACT INFORMATION
Melvin Barnes
Program Manager
Geographic Information Services
Phone: (407) 665-1105
Fax: (407) 665-7412
E-mail
Information Technology Services
1101 E 1st Street
Sanford, FL 32771
Phone: (407) 665-0311
E-mail
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How GIS Works