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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 invaluable for solving many
real-world
problems from tracking delivery vehicles, to recording details of
planning
applications, to modeling global atmospheric circulation.
Geographic References
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. An automated process called geocoding is used
to create explicit geographic references (multiple locations) from
implicit
references (descriptions such as addresses). These geographic
references
allow you to locate features, such as a business or forest stand, and
events,
such as an earthquake, on the Earth's surface for analysis.
Vector
and Raster Models
Geographic information systems work with two fundamentally different
types of geographic models--the "vector model" and the "raster
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 accessibility costs for hospitals.
The raster model has evolved to model such continuous
features. A raster
image comprises a collection of grid cells rather like a scanned map or
picture. Both the vector and raster models for storing geographic data
have unique advantages and disadvantages. Modern GISs are able to
handle
both models.
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