U.S. Geological Survey Open-File Report 2010-1119
The National Assessment of Shoreline Change: A GIS Compilation of Vector Shorelines and Associated Shoreline Change Data for the New England and Mid-Atlantic Coasts
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This section describes the methods used to compile shoreline data and calculate rates-of-change for the New England and Mid-Atlantic coasts. Shoreline DataHistorical shoreline data were acquired from multiple sources and have been documented in the shoreline attribute table and metadata files made available in this report. Shoreline data were digitized from National Oceanographic and Atmospheric Administration (NOAA) Topographic Sheets (T-sheets) and aerial photographs. The dates of historical shorelines range from the mid-1800s to 2000s. A modern shoreline derived from lidar (light detection and ranging) topographic surveys is from data collected in 1997 and 2000. The lidar data were collected by the USGS, in collaboration with the National Aeronautics and Space Administration (NASA). The data used in this study are part of the 2000 Fall East Coast Airborne Lidar Assessment of Coastal Erosion (ALACE) Project.The historical shorelines are based on an interpreted high water line (HWL) that was digitized from topographic maps and aerial photographs. The shorelines extracted from lidar data are referenced to the mean high water line (MHW), which is a different shoreline proxy than the HWL. The HWL shorelines are consistently landward of the MHW shorelines, and the offset between the two shoreline proxies can be considered a bias. This bias value varies alongshore and is documented in an accessory table associated with the lidar shoreline. These values were used in the rate-calculation process to reconcile the offsets between the HWL and MHW shorelines. Lidar data were not available for all sandy beaches along the New England and Mid-Atlantic coasts; gaps exist for the south shore of Cape Cod and the north shore of Cape Cod from Sandwich to Provincetown. When lidar data become available for these gaps, the shoreline changes will be analyzed and provided as online updates to this report and in future reports. For additional information regarding the shoreline-compilation methods and measurement uncertainties and a summary of the results, please refer to the full report: The National Assessment of Shoreline Change—Historical Shoreline Change along the New England and Mid-Atlantic Coasts (Hapke and others, 2010). Calculation of Shoreline Change RatesRates of shoreline change were generated within ArcMap version 9.3 using the Digital Shoreline Analysis System (DSAS) version 4.1, an ArcGIS tool developed by the USGS (Thieler and others, 2009). The tool is a freely available application designed to work within the ESRI ArcGIS software. DSAS is used to generate orthogonal transects starting from a reference baseline and intersecting the shoreline positions at 50-m intervals. The distance measurements between the transect/shoreline intersections and the baseline are then used to calculate the rate-of-change statistics. A linear regression method was used to calculate rates of change over the long term (about 100 yr), and an end point rate calculation was used to capture potential changes in trends or rates over the short term (about 30 yr).End-point Rates (Short-term)Short-term rates of shoreline change were calculated at each transect using an end point rate calculation between the shoreline position from the time period 1971–84 and the modern, lidar-derived shoreline (1997–2000) to provide an approximate 30-yr short-term rate. The end point rate is calculated by subtracting the difference in shoreline position between the two survey years and dividing it by the time between surveys to give a rate in meters per year. The end point rate was not assumed to be linear between the two survey years; this rate simply represents the net change between the two shorelines, annualized to facilitate comparisons with long-term rates found through linear regression. The short-term transect metadata files provide descriptions of the two fields associated with the end point rate calculation, and additional information can be found in Section 7 of the DSAS user guide.Linear Regression Rates (Long-term)Long-term rates of shoreline change, in meters per year, were calculated at each transect as the slope of the linear regression through all shoreline positions from the earliest (1800s) to the most recent (generally the lidar-derived shoreline). Data for a minimum of four available shoreline-survey years were required at each DSAS transect for calculating long-term rates, one of which must be the lidar-derived shoreline. The linear regression method of determining shoreline-change rates was based on an assumed linear trend of change between the earliest and latest shoreline dates. Data for shoreline areas where such a linear trend did not exist and shoreline-change rates have not remained constant through time would produce a poor linear fit to the data with a higher reported uncertainty. The long-term transect metadata files provide descriptions of the four fields associated with the linear regression rate calculation, and additional information can be found in Section 7 of the DSAS user guide.To view files in PDF format, download a free copy of Adobe Reader. |
