NORCAL_BIASVALUES - Northern 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（United States Geological Survey，USGS）通过整合既有历史海岸线数据库中的岸线位置数据，并生成历史与现代岸线资料，构建了一套完整的数字化矢量岸线数据库。岸线按州进行整合，且通常对应四个时间段之一：19世纪、1920年代—1930年代、1970年代以及1998年—2002年。研究团队借助数字岸线分析系统（Digital Shoreline Analysis System，DSAS）3.0版本——一款用于计算岸线变化的ArcGIS扩展模块——在地理信息系统（GIS）中计算了长期与短期的岸线变化速率，相关方法详见文献：美国地质调查局开放文件报告2005-1304，作者为Thieler, E.R.、Himmelstoss, E.A.、Zichichi, J.L.与Miller, T.M.。由历史资料衍生的岸线矢量（前三个时间段）代表测量时的高水位线（high water line，HWL），而现代岸线（最后一个时间段）则代表平均高水位线（mean high water line，MHW）。将岸线定义从基于代理变量、未受高程基准约束的物理特征（HWL），更改为由高程等值线定义的基准岸线（MHW），这一转变对岸线位置的推断变化与岸线变化速率计算均具有重要意义。这种“代理-基准偏移”在沿岸平均岸线变化速率时尤为关键。由于该偏移始终偏向同一方向，表观岸线变化速率偏移相关的误差在平均过程中无法抵消，因此量化该偏差以修正速率偏移至关重要。本报告中呈现的岸线变化速率，已通过校正该代理-基准偏差完成计算。