Beach profile data for the Elwha River Delta, 2017-03-31

Data were collected using GNSS survey methods, with a differential GPS operating in Real Time Kinematic (RTK) mode. Data from prior to 2013 were typically collected with a Magellan ProMark 3 traditional RKT-DGPS system (i.e. local base station and rover), with the base station sited on survey control markers installed in 1996, with survey control coordinates referenced to NAD83(CORS91). Starting in 2013 survey data were typically collected with an AshTech ProMark 200 RTK-DGPS system connected to the Washington State Reference Network. Survey data collected between January and November 2013 are referenced to NAD83(CORS96), and after November 2013 to NAD83(2011).
Vertical data for surveys in 2012 and 2013 are referenced to NAVD88, presumably using Geoid96 (the survey control documentation does not specific a geoid). For all subsequent surveys the vertical data are referenced to NAVD88(Geoid09). No conversion were applied to these data to account for variations in horizontal or vertical coordinate system adjustments through time, but an error analysis suggests a standard deviation for the elevation data of between 0.03 and 0.05 m across the entire sampling period (2011-2018).
All survey data were collected with the GNSS system mounted on a 2.05 m rover pole, held level as a transect line was traced in a cross-shore orientation on the beach.
The associated text files include the horizontal (HRMS) and vertical (VRMS) root-mean-square errors estimated by the GNSS system, as well as the RTK-DGPS status reported by the GNSS system at the time each point was collected. Times are referenced to local Pacific time (either PST or PDT).

本数据集采用全球导航卫星系统（Global Navigation Satellite System, GNSS）测量方法采集，所用差分GPS系统工作于实时动态（Real Time Kinematic, RTK）模式。2013年之前的测量数据通常采用麦哲伦ProMark 3传统RTK-DGPS系统采集（即配置本地基准站与流动站），基准站架设于1996年布设的测量控制点上，测量控制点坐标参考NAD83(CORS91)坐标系。自2013年起，测量数据通常采用与华盛顿州参考网络相连的AshTech ProMark 200 RTK-DGPS系统采集。2013年1月至11月间采集的测量数据参考NAD83(CORS96)坐标系，2013年11月之后的数据则参考NAD83(2011)坐标系。

2012年与2013年的测量高程数据参考北美垂直基准面1988（North American Vertical Datum of 1988, NAVD88），推测采用大地水准面96（Geoid96）（测量控制文档未明确指定所用大地水准面模型）。后续所有测量的高程数据均参考NAVD88(Geoid09)。本数据集未针对不同时期水平或垂直坐标系的调整差异进行坐标转换，但误差分析显示，2011-2018年整个采样周期内，高程数据的标准差介于0.03至0.05米之间。

所有测量数据均通过安装在2.05米高流动杆上的GNSS系统采集，测量时将流动杆保持水平，沿海滩垂直于岸线的方向布设测线并行进采集数据。

配套的文本文件包含GNSS系统估算的水平均方根误差（Horizontal Root-Mean-Square Error, HRMS）与垂直均方根误差（Vertical Root-Mean-Square Error, VRMS），以及每个测点采集时刻GNSS系统报告的RTK-DGPS工作状态。时间参考本地太平洋时间，涵盖太平洋标准时间（PST）与太平洋夏令时（PDT）。