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.
The uncertainty values above do not exactly match those outlined in the average uncertainy table of the corresponding report. In the table, the uncertainty of the high water line (HWL) is incorporated, whereas the uncertainty values above are for individual shoreline positions within each time period. For example, an individual shoreline from 1926 has an uncertainty of 10.8 meters but when the HWL uncertainty from the shorelines_uncertainty.dbf is taken into account, the value increases to 11.7 meters.
Note: Some shorelines were assigned higher uncertainty values than 10.8 meters for the time period 1800s-1950s in Washington. Four shorelines from 1887 have values of 13.25. These differences are for T-sheets that warranted higher values in the uncertainty component involving 'inaccurate location of control points due to distortion or cartographic error.' For more information on the breakdown of T-sheet uncertainty components please refer to Crowell et al. (1991).
Crowell, M., Leatherman, S.P., and Buckley, M.K., 1991. Historical Shoreline Change: Error Analysis and Mapping Accuracy. Journal of Coastal Research: v.7, n.3, pp.839-852.
Repeating this procedure at successive profiles generated a series of X,Y points that contain a lidar positional uncertainty, a bias, and a bias uncertainty value.
Ruggiero, P. and List, J.H., 2009. Improving Accuracy and Statistical Reliability of Shoreline Position and Change Rate Estimates. Journal of Coastal Research: v.25, n.5, pp.1069-1081.
Stockdon, H.F., Sallenger, A.H., List, J.H., and Holman, R.A., 2002. Estimation of Shoreline Position and Change using Airborne Topographic Lidar Data: Journal of Coastal Research, v.18, n.3, pp.502-513.
This process step and all subsequent process steps were performed by the same person - Meredith Kratzmann.
The point feature class was converted to a polyline-M file by connecting adjacent profile points to form a vector shoreline feature using ET GeoWizards (v.9.8) > Convert > Point to Polyline Z (M). The lidar profile ID value was stored as the M-value. The lidar vector shoreline was then converted to a route using ArcToolbox v9.3 Linear Referencing Tools > Create Routes. Parameters: Input Line Features = polyline M file; Route Identifier Field = ET_ID (created by ET GeoWizards when constructing polyline-M field), Measure Source = Length, default values for rest. The unique cross-shore profile ID is stored as the M-value at each vertex.
The route was calibrated using ETGeoWizards (v.9.8) > LinRef tab > Calibrate routes with points. Parameters: Attribute Field selected, Point layer = original point shapefile, Route ID field = RouteID, Measure Field = ID (lidar profile ID), Input search tolerance = 40, Interpolation between points checked, Recalculate measures using calibration points and shortest path distance between vertices checked, every other option unchecked. The calibration assigns interpolated measure values from the start to the end of the route based on the known profile IDs stored at each vertex. This profile ID is used as the common attribute field between the route file and an uncertainty table (WA_shorelines_uncertainty.dbf) storing the lidar positional uncertainty, the bias correction value, and the uncertainty of the bias correction for each point of the original lidar data.
During the rate calculation process DSAS uses linear referencing to retrieve the uncertainty and bias values stored in the associated table. The calculation of these values is explained in detail in the full report, National Assessment of Shoreline Change for the Pacific Northwest Coast, cross-referenced in this metadata file. Please refer specifically to the methods section titled "The Proxy-Datum Bias Correction between HWL and MHW shorelines."
For a detailed explanation of the method used to convert the lidar shoreline to a route, please refer to "Appendix 2- A case study of complex shoreline data" in the DSAS user guide:
Himmelstoss, E.A. 2009. "DSAS 4.0 Installation Instructions and User Guide" in: Thieler, E.R., Himmelstoss, E.A., Zichichi, J.L., and Ergul, Ayhan. 2009. Digital Shoreline Analysis System (DSAS) version 4.0 - An ArcGIS extension for calculating shoreline change: U.S. Geological Survey Open-File Report 2008-1278. <http://woodshole.er.usgs.gov/project-pages/dsas/version4/images/pdf/DSASv4_2.pdf>
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