Database Reference
In-Depth Information
Positioning System) unit so that you have no excuse for ever getting lost. This is an
application of geographic databases.
Here is a random sample of the types of applications:
•
Vehicle navigation
•
Distribution of households by demographics
•
Market analysis by region
•
Consumption of utility services
•
Distribution of economic indicators
•
Pollution studies
•
Water resource management
•
Wildlife studies
•
Soil evaluation studies
•
Landscape surveys
•
Parks management
•
Crop and produce yield analysis
Geographic Data Formats
Although geographic data cover a broad range of
objects, they comprises two types. First, geographic data contain information about
location or spatial objects. These are rivers, lakes, oceans, land masses, boundaries,
regions, cities, towns, and so on. Next, geographic data contain information associ-
ated with these spatial units. For example, geographical data contain spatial infor-
mation about the regions in the north as well as the annual rainfall statistics for
these regions.
Let us formalize these two data formats in geographic databases.
Vector format
Vector data are created from geometric objects such as points, lines,
triangles, squares, and polygons. These are used to represent objects in two dimen-
sions. For example, a river may be represented as a series of line segments, a region
as a combination of polygons, and an exit on the highway as a point. For topologi-
cal information such height and vertical contours, three-dimensional geometric
objects such as cylinders, spheres, or polyhedrons may be used.
Raster format
Raster data represent information associated with spatial data.
Raster data constitute an array or collection of points. Each point represents a value
for the information being represented. Suppose that a region is represented by a
polygon—a vector format. Now you want to show the income distribution in
the region. That would be done with the raster format. If a point in the raster
format represents annual income level of 25,000 dollars, then you would need four
points to represent 100,000 dollars at the appropriate place in the region. Raster data
is kept as
n
-dimensional arrays where each entry is a unit representing an attribute.
Spatial Query Types
How do you use geographic databases? How do you
retrieve information from these databases? We are not discussing the different
indexing techniques needed for geographic databases. B-tree and bitmapped
indexes cannot apply here. However, let us just cover a few types of queries in a
geographic database system.
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