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Layer: block_group20_seattle (ID:46)

View In:   Map Viewer

Name: block_group20_seattle

Display Field: NAMELSAD

Type: Feature Layer

Geometry Type: esriGeometryPolygon

Is View: true

Is Updatable View: true

Source Schema Changes Allowed: true

Sources

Description: Polygons than include Seattle, land extending north, south, and east of citylimits and waterbodies. Additional Notes Extends north to the County line; south to the vicinity of SeaTac, and far enouth east to see Mercer Island, Clyde Hill, and Inglewood. This coverage reflects the visual interface between land and water based upon our 1993 ortho photos. It essentially follows the 8 foot contour line, except where the ortho offered further clarification. That 8 foot contour line matches closest to what NAVD88 shows as "mean high water" (see official definition below) at 7.97 feet. MEAN HIGH WATER (MHW): "A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch." Source: http://co-ops.nos.noaa.gov/mapfinder/mhw.html "The MHW line along the tidal shoreline is represented by the contour line that is 8.0' above the NAVD 88. The MHW elevation is not necessarily an equipotential (level) surface, but the variation from a level surface is not significant for this project. The determination of MHW is based on 18 yrs of tidal data and represents the latest official National Tidal Epoch of 1960-1978. The Seattle Tide Station (Sta No 944-7130) is located at the downtown ferry terminal." Source: From paper by Nick Bodnar 10/18/1993 Some other definitions regarding these shorelines (from Nick Bodnar, 10/18/1993): LAKES UNION AND WASHINGTON SHORELINE (18.6' above NAVD 88) The approximation of "ordinary high water"; the level of Lk Washington varies from 17.0'-18.6' during the year; the max lake elevation is 18.8' as controlled by Corps of Engineers (COE) at Hiram Chittenden Locks. The NAVD 88 lake elevations have been converted from the U.S. Engineers Datum. This zero of NAVD 88 is 3.25' above the Locks Datum at the common bench mark P-7. The shoreline contour of 18.8' above NAVD 88 should be used from the Locks, upstream through Lk Union and Lk Washington. The COE has performed various studies that show the level of Lk Wash to approximate an equipotential surface that can be represented by a single contour elevation for mapping purposes. OTHER LAKES The shoreline around Green Lake and all other lakes are delimited by the water/land interface as seen on the aerial photography.

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Max Record Count: 2000

Supported query Formats: JSON, geoJSON, PBF

Use Standardized Queries: True

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Last Edit Date: 4/30/2024 9:39:13 PM

Schema Last Edit Date: 4/30/2024 9:31:47 PM

Data Last Edit Date: 4/30/2024 9:31:47 PM

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