There is no widely accepted standard for analyzing shoreline change. Existing shoreline data measurements and rate calculation methods vary from study to study and prevent combining results into state-wide or regional assessments. The impetus behind the National Assessment project was to develop a standardized method of measuring changes in shoreline position that is consistent from coast to coast. The goal was to facilitate the process of periodically and systematically updating the results in an internally consistent manner.
Rates of long-term and short-term shoreline change were generated in a GIS using the Digital Shoreline Analysis System (DSAS) version 4.2. DSAS uses a measurement baseline method to calculate rate-of-change statistics. Transects are cast from the reference baseline to intersect each shoreline, establishing measurement points used to calculate shoreline change rates.
Geo-referenced T-sheets were loaded in ArcMap version 9.3. ArcMap was used to transform received Geographic Projected data into the Universal Transverse Mercator (UTM) projection with the North American Datum of 1983 (NAD83) datum for digitizing and vector analysis. A verification of the T-sheet shoreline was carried out where possible using control marks or physical shoreline features present on the T-sheet and a reliable current image database. Where verification failed, T-sheets were re-rectified in PCI Geomatics Orthoengine version 10.1 software using ground control points on existent control stations and identifiable shoreline features. In all cases, shoreline feature verification produced a higher quality data product.
Many T-sheet products used in this study were re-rectified in PCI to correct significant errors associated with incorrect projection datum definitions. Such errors would have otherwise rendered the sheets unusable. T-sheets are georeferenced using polynomial mathematical models in PCI with RMS errors < 4 m. Rectification of T-sheets is also verified by overlaying them on aerial photomosaics to compare their fit to rocky shoreline and other unchanged geological features.
Earlier aerial photographs are orthorectified and mosaicked in PCI Geomatica Orthoengine using 2005-2008 and 1998 (west and north Maui) aerial orthophoto mosaics as master images (for collecting Ground Control Points (GCPs)) and high resolution (5m -horizontal, submeter vertical) DEMs to reduce displacements caused by lens distortion, Earth curvature, refraction, camera tilt, and terrain relief; usually achieving Root Mean Square (RMS) positional error < 2 m.
Shoreline layers were saved from .pix (PCI Geomatics geographic data format) to shapefile (.shp) to allow for analysis in ESRI ArcGIS software using Digital Shoreline Analysis System (DSAS). Shoreline layers from all sources were merged to produce a single shoreline shapefile for the coastal region. The final shoreline shapefile was coded with 4 attribute fields (ID, Shape, Date, and Accuracy) required for the DSAS, which was used to calculate shoreline change rates. The attributes reflect the source of the data and the original survey date.
This and all following process steps performed by Bradley M. Romine, University of Hawaii Coastal Geology Group.
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