Digital elevation models (DEMs) of the Elwha River delta, Washington, July 2017

This portion of the USGS data release presents digital elevation models (DEMs) derived from bathymetric and topographic surveys conducted on the Elwha River delta in July 2017 (USGS Field Activity Number 2017-638-FA). Nearshore bathymetry data were collected using two personal watercraft (PWCs) and a kayak equipped with single-beam echosounders and survey-grade global navigation satellite systems (GNSS) receivers. Topographic data were collected on foot with survey-grade GNSS receivers mounted on backpacks. Positions of the survey platforms were referenced to a GNSS base station placed on a benchmark with known horizontal and vertical coordinates relative to the North American Datum of 1983 (CORS96 realization) and North American Vertical Datum of 1988. The final data were projected in Cartesian coordinates using the Washington State Plane North (meters) coordinate system. A total of 1,270,212 individual elevation points were collected within the survey area between July 20 and July 23, 2017. DEM surfaces were produced from all available elevation data using linear interpolation. Two separate DEMs were constructed. A DEM was produced that covered the entire survey area (approximately 511 ha) with 5-m horizontal resolution. A second DEM with 1-m resolution was produced that covered the river mouth and adjacent areas (approximately 131 ha). The DEMs were created by interpolating between measurements as much as 50 meters apart. For this reason, we cannot evaluate the accuracy of each point in the DEM, only the measurements it is based on. The estimated vertical uncertainties of the bathymetric and topographic measurements are 12 and 5 cm, respectively. Digital data files for each DEM are provided in ESRI ARC ASCII (*.asc) format.

本部分美国地质调查局（United States Geological Survey, USGS）数据发布内容包含2017年7月在埃尔克河三角洲开展的水深与地形测量所生成的数字高程模型（digital elevation models, DEMs），对应美国地质调查局野外活动编号2017-638-FA。近岸水深数据通过两艘个人水上交通工具（personal watercraft, PWCs）与一艘皮划艇采集，上述平台搭载单波束测深仪与测量级全球导航卫星系统（global navigation satellite systems, GNSS）接收机。地形数据则通过背负式测量级GNSS接收机徒步采集。测量平台的位置以设置在基准点上的GNSS基准站为参照基准，该基准点具备相对于1983北美大地基准（CORS96实现版本）与1988北美垂直基准的已知水平与垂直坐标。最终数据采用华盛顿州平面坐标系北分区（米制）的笛卡尔坐标系进行投影。2017年7月20日至23日期间，在测量区域内共采集到1,270,212个独立高程点。研究团队利用所有可用高程数据通过线性插值生成DEM表面，本次共构建两套独立DEM：一套覆盖全测量区域（约511公顷），水平分辨率为5米；另一套覆盖河口及邻近区域（约131公顷），水平分辨率为1米。两套DEM均通过对间距最大可达50米的原始测量点进行插值生成。鉴于此，无法对DEM内单个点位的精度进行评估，仅可对其依托的原始测量数据开展精度验证。本次水深测量与地形测量的估算垂直不确定度分别为12厘米与5厘米。每套DEM的数字数据文件均以ESRI ARC ASCII（*.asc）格式提供。