Checking VDatum Offshore with Bottom Mounted Pressure Gauge Geodetically Referenced with GNSS ASV

The United States National Oceanic and Atmospheric Administration has a Vertical Datum Transformation tool (VDatum) that allows for the conversions between tidal, ellipsoid and orthometric vertical datums. These datums are adopted by different agencies for measurements. One important application of the Global Navigation Satellite System (GNSS) is to facilitate for hydrographic surveying to the ellipsoidal datum and using the VDatum tool to reduce the soundings to the Mean Lower Low Water (MLL W) which is a tidal datum. This has resulted in several studies to determine the regions where these large uncertainties prevail. These uncertainties cannot be absorbed by the IHO vertical uncertainty budget for special order through order 1 b. In the previous studies using USGS coastal water level gauges and an offshore buoy, errors in the Great diurnal range (Gt) tidal datum and the topography of sea surface (TSS) were determined in some of the study areas. For the latter case, repeated vandalism of the surface buoy limited the duration of the water level record. In this study, a novel technique was adopted to reduce the chances of vandalism and obtain a minimum 30-day water level record for tidal datum computation. It involved the use of a bottom-mounted pressure gauge, deployed for more than 30 days, and an autonomous surface vehicle (ASV), equipped with both survey grade and low cost global navigation satellite systems (GNSS), deployed for 7 hours, to tie the sea level data to the ellipsoid. In addition to these datasets, oceanographic parameters such as temperature, salinity and density of the seafloor were measured. The CTD and pressure sensor datasets were used to investigate the temporal evolution of these oceanographic parameters including the variabilities in the oceanographic parameters due to the passage of a tropical storm. U sing the hydrostatic balance equation as well as the ellipsoidal height of the sea surface obtained from the ASV, the ellipsoidal height of the sea surface was estimated. Tidal datums were computed using the sea surface ellipsoidal height by applying the modified range ratio technique as described in [1] using the Dauphin Island NOAA water level station as a primary station. The computed tidal datums were compared to VDatum results to determine the fidelity of the VDatum tool in estimating tidal datums over the study area. The results of this study agreed to VDatum outputs within the published VDatum uncertainty of ± 0.17 m.