The Study uses aerial photographs to derive historical shoreline positions. The aerial photograph collection currently consists of 8 regional sets taken between 1942 and 2001. Additional photographs have been collected for Ocean Shores, WA, Damon Point, WA, Grays Harbor County and Fort Canby State Park.
From aerial photographs to digital shorelines
First, the photos are scanned at 1200 dpi and stored as *.tif images. The next step is to register the photos to the Washington State Plane, South, meters (NAD 83) coordinate system using reference marks and ground control points. The image is then orthorectified, or stretched and warped, based on the control points, and image processing software is used to correct for distortion induced by the camera lens. Once the photos are rectified, they are compiled to create an orthophoto mosaic. The orthophoto mosaic consists of individual photographs joined together to form a single image of a segment of coast.
Shoreline reference features, including the Average High Water Line (AHWL) and the vegetation line, are digitized from the orthophoto mosaic. The littoral cell features a semidiurnal tidal pattern, two high tides and two low tides of unequal height every day. The AHWL is based on the difference between the highest high tide and the last high tide. Shalowitz (1964) defines the location of the AHWL on such coastal systems as falling between the daily high tide lines. If there is evidence of only one line, then AHWL is located slightly seaward of the single drift line. The vegetation line is defined as the location where vegetation increases to more than 50 percent.
The digitizing process is accomplished with the orthophoto mosaic displayed as a backdrop image. The operator traces the features of the beach (e.g., AHWL) based on the appearance of the background image. This process also allows the operator to zoom in to resolve questionable features on the beach. The resulting data is a vector, or line coverage of the AHWL and the vegetation line that can be included in the GIS database and used for shoreline change analysis.
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