The Census Bureau provides data for many geographic areas, including counties, cities (called places by the U.S. Census Bureau), census tracts, block groups, and blocks. Seattle is divided into 131 census tracts and extends slightly into three additional census tracts. Seattle's census tracts are divided into 482 block groups (some partial). The block groups are further divided into 11,512 blocks. Blocks are the smallest geographic areas used by the Census Bureau. Blocks are nested in block groups which are, in turn, nested in census tracts.
The City of Seattle standard projection and datum for geospatial data layers is State Plane Washington North, North American Datum 1983 HARN in units of US feet. All of the GIS layers have been re-projected to the City of Seattle standard projection and coordinate system.