VDATUM the Vertical Datum Transformation Tool Kurt Hess

VDATUM: the Vertical Datum Transformation Tool Kurt Hess Office of Coast Survey • A vertical datum is a reference surface for elevations • NOAA Chart Datum for Depths: Mean Lower Low Water (MLLW) • NOAA Shoreline: Mean High Water (MHW)

RTK/PPK-GPS Hydrographic Surveys Depths are measured “on the fly” relative to ellipsoidal datum (e. g. , WGS 84). Depths need to be converted to chart datum (MLLW). Eliminates need for: -tide gauges during survey -settlement & squat corrections for survey vessel -time-consuming postsurvey processing

Need to Integrate Land Water Datasets with Different Elevation Reference Systems NOAA (Tidal Datums) (Intertidal) USGS (Orthometric Datums) ‘MSL’ Mapped Shoreline MHW Elevations MLLW (MLW) NAVD 88, NGVD 29 NAD 27 NAD 83 Depths MLLW MHW Charted Shoreline

How to convert between these various datums? Vdatum Conversion Software Tidal Datums MHHW MSL MLW MLLW Orthometric Datums NAVD 88 NGVD 29 Ellipsoidal Datums WGS 84 ITRFxxxx NAD 83

DELAWARE BAY PPK-GPS SURVEY, June 2001 VDatum gives a more realistic series of depths values Tide Zones VDatum vs Zoning : ‘Jump’ due to crossing tide zones VDATUM conversion fields

Digital Elevation Models to Study Sea Level Rise in North Carolina NC Land Elevation NOAA Bathymetry Integrated DEM Inundated areas due to a 30 cm sea level rise

Consistently–defined Shoreline LIDAR data transformed from ellipsiodal to the MHW datum using VDatum. NOAA T-Sheet MHW shoreline (1977) 0 m contour is MHW shoreline (1996) - first true consistentlydefined MHW shoreline - can utilize topographic LIDAR data from many sources - can also measure MLLW from bathymetric LIDAR - true interface between bathymetry & topography

VDATUM TODAY Areas where VDatum tool is complete and available to the public

In Summary, VDatum Supports: • Faster, cheaper, more accurate hydrographic survey data • Merging NOAA bathymetric data for GIS and other uses • Integrated bathymetric-topographic DEMs for: • Storm surge and flooding • Port planning and Homeland Security • Sea Level Rise studies • LIDAR coastal elevation data for: • Consistently defined shoreline for NOAA, USGS • NOS charting products • maritime boundaries

Issues • We must educate users about the importance of understanding and using the various vertical datums and reference systems • Establishing VDatum for entire US will require (1) setting priorities for geographic areas and (2) long-term hydro survey planning • Tide and surveyed benchmark data are often lacking or are outdated. Also true for bathymetric data, which are needed for tide modeling • There is a need for many more elevation measurements (e. g. , LIDAR) in the inter-tidal zone