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about gis: CGDB spatial accuracy standards
The City of Seattle’s Central Geographic Data Base (CGDB) is one of the most spatially accurate GIS data bases in the country. However, it is not error free. Spatial accuracy varies within particular areas of the City and in the utility service areas north and south of the City limits. The information below is intended to provide general guidelines pertaining to CGDB spatial accuracy which will help users determine appropriate and inappropriate uses of individual layers and the CGDB as a whole.

The federal government’s “National Map Accuracy Standards” provide accuracy guidelines and standards for published maps. In addition, the Federal Geographic Data Committee (FGDC) has developed a revised “National Spatial Data Accuracy Standard” that was finalized in 1996. These standards follow the lead of the Federal Geodetic Control Subcommittee (FGCS) by using a circular accuracy number at the 95% confidence interval.

Under the current system, to meet the horizontal standards for large scale maps (1:20,000 or larger, 90% of the “well defined” horizontal (X,Y) locations checked must be within 1/30 inch at map scale. The CGDB design scale of 1:1200 (1in = 100ft) fits the definition of a large scale map. This means that to meet the “National Map Accuracy Standards”, 90% of the “well defined” horizontal locations must be within 3.3 ft of their real world location. The accuracy goal for the CGDB is for horizontal map features to be within +- 2ft of their real world location. Although it has never been systematically verified, experience indicates that within Seattle City limits, the CGDB generally meets the +- 2ft goal, and would readily conform to National Map Accuracy Standards (figures below assume conformance). The north and south service areas were produced without the benefit of extensive survey control, and would probably NOT meet the National Map Accuracy Standards.

It is important to note that these standards are in reference to the original map scale. Maps generated at larger scales do not increase in accuracy. Maps produced at other than design scale should be so noted, as shown under item 6 on the attached “United States National Map Accuracy Standards”.

Following is a summary of the National Map Accuracy Standards for the design scale of 1:1200 (1in = 100ft) which is relevant to CGDB data sets.

CGDB Control, Legal and Parcel Layers (within city)
horizontal accuracy 90% of “well defined” horizontal locations within +- 3.3 feet

Topo Layers (contours, spot elev., mass points, breaklines & DTM data)
vertical accuracy

Contours: 90 % of “well defined” locations @ 1/2 contour interval (+- 1 ft)
10% of “well defined” locations @1 full contour interval (+- 2 ft)

Spot Elev: 90% “well defined” locations @ 1/4 contour interval (+- 0.5 ft)
10% “well defined” locations @ 1/2 contour interval (+- 1 ft)

Mass Pts: 90% “well defined” locations @ 1/4 contour interval (+- 0.5 ft)
10% “well defined” locations @ 1/2 contour interval (+- 1 ft)

Breaklines: 90% “well defined” locations @ 1/4 contour interval (+- 0.5 ft)
10% “well defined” locations @ 1/2 contour interval (+- 1 ft)

DTM: 90% “well defined” locations @ 1/4 contour interval (+- 0.5 ft)
10% “well defined” locations @ 1/2 contour interval (+- 1 ft)

Building Outlines
horizontal accuracy 90% of “well defined” horizontal locations within +- 3.3 ft
vertical accuracy +- 1 ft (project specific)

note: In some cases, building outlines overlaid on digital orthophotos will not be co-incident and appear offset from the orthophoto image. This effect is called relief displacement and is most noticeable on tall buildings. Orthophoto images are rectified or corrected at ground elevation. Any features not at ground elevation (eg. tops of buildings) will not appear in the image in their true planimetric location.

Digital Orthophotos
horizontal accuracy 90% of “well defined” planimetric features within +- 3.3 ft

note: When plotted at larger than design scales (design scale: 1in = 100ft),digital ortho images will lose their clarity. The maximum recommended plot scale for CGDB digital orthos is 1in = 50ft.

Summary & Recommendations
The City of Seattle’s Central Geographic Data Base is a high accuracy digital GIS base map. Used appropriately, the CGDB provides many efficient tools and opportunities for utilizing spatial data in new and creative ways. Conversely, the data is readily accessible and easily manipulated, creating a great potential for misuse. To avoid potential problems, one must be aware of limitations in the accuracy of the data and apply prudent judgment in its use. It is the user’s responsibility to verify accuracy and determine if the data is fit for their particular applications or uses. Questions should be directed to Ken Conradi in GIS and Corporate Data.

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© 2001, THE CITY OF SEATTLE, all rights reserved.
No warranties of any sort, including accuracy, fitness or merchantability, accompany this product.
 

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