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., WGS84). Depths need to be converted to chart datum (MLLW). Eliminates need for: -tide gauges during survey settlement & squat corrections for survey vessel time-consuming post-survey processing VDatum also allows NOAA to finally take full advantage of some recent technological advancements, such as real-time kinematic (RTK) GPS, that will improve the efficiency with which new data are acquired and the accuracy of the data. For example, traditional hydrographic surveys have required: - simultaneous real-time tide gauges in order to subtract out the water level signal from the total depth soundings, - settlement and squat corrections of the vertical movement of the survey vessel as it moves; and - time-consuming post-survey data processing. VDatum eliminates the need for all of these, and allows the depth soundings, which are referenced to the ellipsoid with the RTK GPS, to be directly transformed to the Mean Lower Low Water Datum (“on the fly”)

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

Tidal Datums MHHW MHW MSL MLW MLLW How to convert between these various datums? Vdatum Conversion Software Tidal Datums MHHW MHW 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 : VDATUM conversion fields ‘Jump’ due to crossing tide zones

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

NOAA T-Sheet MHW shoreline (1977) 0 m contour is MHW shoreline (1996) 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 consistently- defined MHW shoreline can utilize topographic LIDAR data from many sources can also measure MLLW from bathymetric LIDAR true interface between bathymetry & topography The development of VDatum will also support the use of LIDAR technology to determine a consistent shoreline for the United States. A present problem is that shoreline shown on NOAA’s nautical charts does not match the shoreline shown on U.S. Geological Survey land maps. USGS shoreline is referenced to an orthometric datum and NOAA shoreline is referenced to a tidal datum. Many customers have commented on this inconsistency between NOAA and USGS products. At user workshops they have told us, even before accuracy, they would like to see consistency. In this slide, we see shoreline from the NOAA nautical chart in red superimposed on LIDAR data. The green line is shoreline produced from LIDAR data by transforming it with VDatum to the mean high water datum. This difference in the entrance to Long Branch creek is due to the mangrove swamps. The LIDAR saw through the mangroves and correctly determined the shoreline to be at the zero contour of the elevation data referenced to the Mean High Water datum. Back in 1977, the hydrographic field party put the shoreline at the edge of the mangrove swamp.

Areas where VDatum tool is complete and available to the public VDATUM TODAY VDatum is presently available for small areas of the U.S We have a long way to go! 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