CENCAL_BIASVALUES - Central California Shoreline Bias Values

The USGS has produced a comprehensive database of digital vector shorelines by compiling shoreline positions from pre-existing historical shoreline databases and by generating historical and modern shoreline data. Shorelines are compiled by state and generally correspond to one of four time periods: 1800s, 1920s-1930s, 1970s, and 1998-2002. These shorelines were used to calculate long-term and short-term change rates in a GIS using the Digital Shoreline Analysis System (DSAS) version 3.0; An ArcGIS extension for calculating shoreline change: U.S. Geological Survey Open-File Report 2005-1304, Thieler, E.R., Himmelstoss, E.A., Zichichi, J.L., and Miller, T.M. Shoreline vectors derived from historic sources (first three time periods) represent the high water line (HWL) at the time of the survey, whereas modern shorelines (final time period) represent the mean high water line (MHW). Changing the shoreline definition from a proxy-based physical feature that is uncontrolled in terms of an elevation datum (HWL) to a datum-based shoreline defined by an elevation contour (MHW) has important implications with regard to inferred changes in shoreline position and calculated rates of change. This proxy-datum offset is particularly important when averaging shoreline change rates alongshore. Since the proxy-datum offset is a bias, virtually always acting in the same direction, the error associated with the apparent shoreline change rate shift does not cancel during averaging and it is important to quantify the bias in order to account for the rate shift. The shoreline change rates presented in this report have been calculated by accounting for the proxy-datum bias.美国地质调查局（USGS）通过整合现有历史海岸线数据库中的海岸线点位，并生成历史与现代海岸线数据，构建了一套完整的数字矢量海岸线数据库。该数据集按州级进行整合，海岸线点位通常对应以下四个时期之一：19世纪、1920年代-1930年代、1970年代以及1998年-2002年。研究团队借助数字海岸线分析系统（Digital Shoreline Analysis System, DSAS）3.0版本——一款用于计算海岸线变化的ArcGIS扩展工具，在地理信息系统（Geographic Information System, GIS）中计算了这些海岸线的长期与短期变化速率，相关成果载于美国地质调查局公开文件报告2005-1304，作者为Thieler, E.R.、Himmelstoss, E.A.、Zichichi, J.L.及Miller, T.M.。源自历史数据源的海岸线矢量（前三个时期）对应调查时的高潮线（High Water Line, HWL），而现代海岸线（最后一个时期）则对应平均高潮线（Mean High Water Line, MHW）。将海岸线定义从基于代理指标、未受高程基准约束的物理特征（即HWL），转换为基于高程等值线确定的基准面海岸线（即MHW），对于推断海岸线位置变化及计算变化速率具有重要意义。在沿岸平均化海岸线变化速率的过程中，这种代理基准面偏差尤为关键。由于该代理基准面偏差始终朝向单一方向，因此在平均化过程中，表观海岸线变化速率偏移所对应的误差无法被抵消，因此量化该偏差以修正速率偏移结果至关重要。本报告中呈现的海岸线变化速率，已通过修正代理基准面偏差完成计算。