The "Western Washington Land Cover Change Analysis" report and GIS products available were produced by Maria Fiorella, of Sanborn Mapping Solutions.
The shapefile/layer depicting basins that potentially quailify for use of the lower flow control standard will likely change in the future. Reasons for changing the designations in the Basins_1991Criterion.shp file include: 1) An alternative flow control standard has been justified based upon basin-specific studies; 2) A local government determines that a basin originally designated as potentially qualifying for use of the lower flow control standard, does not qualify for that standard because it was not at 40% total impervious area as of 1985; 3) A local government provides a more accurate analysis of the land cover condition for a basin; 4) A stream that was not initially designated as potentially qualifying because of channel instability is subsequently designated because of new infomrmation concerning the stream channel; 5) A local government provides a more accurate analysis of the land cover condition for a basin.
In regard to the underlying data set for the map, the Department does not have any plans to make alterations to the existing data. The Department also has not made any commitments to update the land cover analysis to represent a more recent land cover condtion.
Time Period for which the data is relevant: The production of the map relied on interpretation of satellite images taken between July 7, 1991 to September 23, 1991. The land cover information reported for 1996 relied on interpretation of satellite images taken between September 18, 1995 to August 21, 1996. The land cover information reported for 2001 relied on interpretation of satellite images taken between October 16, 1999 to February 26, 2002.
Ecology used one of the report's outputs, total impervious area by basin in 1991, to determine which areas potentially qualify for use of the existing land cover conditioin as the flow control target for new and redevelopment projects.
For the Basins_1991 Criterion.shp file, we computed an area weighted average by basin based upon Shape_Area and Imp_1991. If weighted average >40%, then use existing land cover; If weighted average < 40% TIA, basin does not qualify for less stringent flow control standard.