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-----------------------------------------The creation of the basin model was a painstaking process based on a LiDAR DEM gridded at six-foot postings, the 2006 NAIP orthophoto series at 18-inch pixel size, previously mapped rivers and basin boundaries, and mapped tide gates. The first step was to reproject the North Puget Sound LiDAR Survey from stateplane coordinates to UTM NAD27 at 2-meter resolution. The DEM was filled, flow directions were calculated, and a basin boundary was determined. The flows that cross the edges of each 150-meter cell were calculated, yielding an optimal 150-meter flow direction grid.
Normally, one would be satisfied to generalize a DEM from a 2-meter source to a 150-meter result, but low slopes and numerous anthropomorphic features made considerable work necessary. The process of filling sinks sometimes encounters large spurious sinks, beneath which flow direction information is lost. We therefore repeatedly clipped numerous subsets of the two-meter DEM, filling sinks, and calculating basin boundaries. When there were discrepancies between sources, we examined DEMs and photos at maximum resolution. After settling on the Basin boundary, we mapped out flowdirection arrows and modified flow directions where appropriate.
lidarshed_b.zip shapefile of the boundary. Parts of the boundary which were automatically derived from the six-foot DEM have vertices at six-foot intervals, while freehand interpretations have fewer vertices.
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