Bathymetry data for Lake Sammamish, Washington collected during USGS field activity 2021-656-FA

The data were collected with a SWATHplus-M 234-kHz interferometric side-scan sonar system pole-mounted on the U.S. Geological Survey (USGS) R/V San Lorenzo. During the mapping mission, an Applanix POS-MV (Position and Orientation System for Marine Vessels) was used to position the vessel during data collection, and it also accounted for vessel motion such as heave, pitch, and roll. The POS-MV also recorded position and vessel motion parameters for later post-processing. Sound-velocity profiles were collected throughout the survey days with a Castaway sound velocimeter. Post-processed POS-MV Smoothed Best Estimate of Trajectory (SBET) files were merged with the survey line files using BathySwath software and then the line files were imported into SonarWiz software. Soundings were cleaned for obvious errors and gridded into a 2-m base surface. The base surface was exported as an Erdas Imagine file in UTM-10, WGS-84 coordinates vertically referenced to the WGS84 ellipsoid. The data were transformed from WGS-84 to NAD-83 horizontal coordinates using ArcGIS Pro ArcTool raster projection command WGS_1984_(ITRF08)_To_NAD_1983_2011. NOAA's on-line VDATUM transformation tool states there is a +23.688m difference between WGS-84 and NAVD-88 elevations (vertical uncertainty of 0.06m) at a point in the center of the lake. A value of 23.688m was added to the original bathymetry grid to transform from WGS-84 elevations to NAVD-88 elevations. The NAVD-88 bathymetry grid was then exported as a GeoTIFF file in UTM zone 10, NAD-83(2011) coordinates, relative to NAVD88.