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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.

It has now 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.

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.

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.

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.


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.