This portion of the USGS data release presents topography data collected during surveys performed on the Elwha River delta, Washington, in May 2011 (USGS Field Activity Number W-04-11-PS). Topography data were collected on foot with global navigation satellite system (GNSS) equipment mounted on backpacks operating in Real-Time Kinematic (RTK) mode. A total of 3 GNSS backpacks, each equipped with a dual-frequency GNSS receiver and handheld data collector, were used during the survey. Differential corrections were transmitted at 1-Hz intervals by a VHF radio to the backpack GNSS receivers from 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.
Prior to data collection, vertical distances between the GNSS antenna and the ground were measured using a tape measure for each topographic surveyor. Hand-held data collectors were used to log raw data and display navigational information allowing surveyors to navigate along predefined survey lines spaced at 25- to 50-m intervals along the delta. Profiles were surveyed from the landward edge of the primary dune over the beach foreshore, to wading depth on the same series of transects as nearshore bathymetric surveys that were conducted during the same time period. Additional topography data were collected between transects to better characterize morphologic variability within the study area.
The estimated vertical uncertainty of the topographic measurements is 5 cm. The final point data are provided in comma-separated text format and are projected in Cartesian coordinates using the Washington State Plane North, meters coordinate system.