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Introduction to GIS at The City of Seattle
The
history of Seattle's Geographic Information System (GIS) spans 18 years. Evolving from a
small installation in the former Seattle Engineering Dept., GIS capabilities are now
firmly entrenched within the daily business functions of at least 6 City Departments. The
City's GIS was originally built primarily to improve the way the City manages and operates
its utility infrastructure. |
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matured to a point where it can support complex business functions in most of the City's
Departments. GIS data and capabilities are used at the City to inform decision makers and
planners, help deliver services to the public, dispatch Police and Fire personnel, and
manage City real estate. |
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| What is GIS? |
| The
elements that make up a GIS combine to provide a powerful tool that enables the user to
analyze and further understand the spatial relationships among things that exist and occur
in a given location. The results of geographic analysis can be communicated with maps,
reports, or both. |
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| Elements of a GIS include: |
A computerized map (layers)
made up of entities (points, lines, polygons) that have geographic positions on the
earth's surface as well as relationships to all the other entities within the layer
(topology). Layers are generally distinguished from one another by their subject matter
(i.e. streets, roads, water bodies).
- A database of descriptors
(attributes) related to the map entities.
- A set of software tools that perform
complex spatial analysis operations.
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| What is the Central Geographic Data Base (CGDB)? |
| The 5 GIS
databases (layers) of the Central Geographic Data Base (CGDB) are the foundation for the
City of Seattle's automated geographic systems environment. These are the shared layers to
which all other City GIS layers are spatially registered. The CGDB was built to meet an
accuracy requirement of +/- 1 to 2 feet.
The underlying base layers of the CGDB consist of the Public Lands Survey System grid
(PLS), Legal and Control layers. These layers were constructed from a combination of
existing coordinate information, GPS and inertial surveys, photogrammetric densification
and COGO calculations based on plat information and other survey data.
Spatial accuracy was preserved in the Parcel Layer and Street Network Database by
deriving parcel boundaries and street centerlines from existing CLDB graphics and survey
control.
The Orthophoto, Topography and Building Outline layers were independantly developed
from newly established survey control, using photogrammetric compilation techniques.
The result is one of the most spatially accurate set of GIS
base layers in the country. |
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| Using GIS |
| A GIS does
not store a map in any conventional sense; nor does it store a particular image or view of
a geographic area. Instead, a GIS stores the data from which you can create the desired
view, drawn to suit a particular purpose. Data sets available through GIS
can be combined to make a wide variety of maps and/or perform analysis. However, data
cannot be combined arbitrarily. For maps there are limitations on output size, scale, and
readability. IT Storefront staff can lend their expertise to help you get the product
that will best suit your needs whether it be a map, formatted data, a mailing list, or the
results of geographic analysis. |
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