Digital elevation models (DEMs) of the Elwha River delta, Washington, July and August 2023

Digital elevation models (DEMs) are created by interpolating between elevation measurements as much as 50 meters apart, for this reason we cannot evaluate the accuracy of each point in a DEM, only the original data from which it is generated. An estimated vertical accuracy of 5 cm for the topographic GNSS backpack survey platforms was estimated by combining uncertainty in the base station coordinates and the manufacturer reported vertical accuracy of 1.5 cm + 1 ppm for differential corrected kinematic data. Additional uncertainty in the final computed elevations in the bathymetry data is related to unmeasured variability in the speed of sound used to compute depths from bathymetric soundings and, thus, are depth dependent. Based on analysis of all available sound velocity casts, we estimate the uncertainty in the final elevations to be 0.6 percent of water depth. The total vertical uncertainty is calculated using the standard equation sigmat=v(a^2+(b×d)^2), where sigmat is the total vertical uncertainty, a is the uncertainty of the GNSS vertical position, b is the depth-dependent uncertainty factor (0.006), and d is water depth (International Hydrographic Organization, 2008). Maximum water depths surveyed by the personal watercraft are approximately 18 m, yielding a maximum total vertical uncertainty of 12 cm.