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CENTRAL GEOGRAPHIC DATA BASE
The 5 layers which comprise the Central Geographic Data Base (CGDB) provide the foundation for the City of Seattle's automated geographic information system (GIS) environment.

Its primary base layer, the Legal Layer, was 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 graphics and survey control. The Orthophoto and Topography layers were independently 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.


UTILITY LAYERS

Sewer/Drainage Infrastructure

The GIS layers which together create a detailed depiction of the City's sewer & drainage infrastructure were created between 1993 and 1995 using records stored on side sewer cards and CAD drawings. Side sewer cards and drawings were first scanned, then registered to known coordinates derived from base layers within the CGDB. Attributes were assigned to each feature using existing paper maps as a source. The resulting files were annotated with additional information to enhance useability and to accommodate specialized map products.
Water Distribution System
Using the CGDB Legal Layer as the spatial foundation, the GIS layers which depict the City's highly complex water distribution system were created using existing paper maps, construction design documents, and 'as-built' plans as the primary sources. System components were digitalized and registered to features within the CGDB before attributes were individually assigned. Accuracy, currency and completeness of the water system GIS layers depends on the timely receipt of 'as-built' plans by SPU/GIS staff.

THEMATIC LAYERS
SPU/GIS stewards a large repository of thematic data sets which have their source in other City departments, County, State and Federal agencies, private businesses and community organizations. Much of this data has been compiled for use on special projects and is not regularly maintained by the SPU/GIS staff. Most data sets which are maintained by GIS staff in other City departments are routinely transmitted to SPU/GIS subsequent to an update or other change.
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