Tide Height used for correcting multibeam sonar bathymetry to the NAVD88 vertical datum for the LIS4A multibeam surveys in 2022

The water height records used to process the multibeam data of the LIS4A surveys were based on the NOAA water-level station at New Haven, CT (Station 8465705; reported referenced to MLLW), and the LIS4A surveys were centered on this water-level station. Vessel elevation was measured during the survey by ship-mounted GNSS satellite receivers that used corrections from Fugro geostationary satellites (Seapath 130-5+; referenced to WGS84) or from land-based CORS/RTN stations (Trimble BX982; referenced to NAD83). The height records in these three data files were converted to NAVD88 (approximately the mean tidal elevation) using the NOAA software program VDatum (www.vdatum.noaa.gov) and compared. The three records were in agreement for all three surveys although the NOAA water-level record was more continuous than the GNSS records and so was used as the basis for the water-level correction. Since the multibeam transducer is attached to the hull of the vessel, a correction to the water-level record may be needed since the vessel will float higher in the water as fuel is used during the survey. We were able to measure this vessel rise during the R/V Connecticut survey which did not refuel during the survey and a correction for fuel use is incorporated in the water-level record for that survey (as denoted by "FUEL" in the file name). In contrast, R/V Seawolf refueled halfway through both the September and November-December surveys and no fuel correction could be determined for those surveys. When these water-level files are used to process the multibeam depth data, the resulting depths will be relative to NAVD88 (North Atlantic Vertical Datum 1988) which is about 1 m higher than the normal charting datum of MLLW (Mean Lower Low Water). Thus the water depths reported for the LIS4A surveys suggest that the water is 1 m deeper than is reported on the NOAA charts. This is false, and, thus, LIS4A depth data are not to be used for navigation. The tide files are in ASCII format and contain three columns: Date (yyyy/mm/dd), time (hh:mm), and water level (meters relative to NAVD-88). This study was supported by Contract Number NORTHE8361-001 issued to Stony Brook University by DEEP - Land & Water Resources Division (LWRD), 79 Elm Street, Hartford, CT 06106-5127. This project has been funded wholly or in part by the United States Environmental Protection Agency under assistance agreement LI-00A00579-0 to the Connecticut Department of Energy and Environmental Protection (DEEP). The contents of this document do not necessarily reflect the views and policies of DEEP or the State of Connecticut or of the Environmental Protection Agency, nor does DEEP or the EPA endorse trade names or recommend the use of trade names or recommend the use of commercial products mentioned in this document.