VDatum (MiIlbert and Hess, 2001 (pdf)) is a tool for the transformation of elevation data from one vertical datum into another. Such transformations are necessary when data from diverse sources are to be combined or compared. Informally, a datum can be considered the arbitrary zero level of the vertical coordinate of geospatial data. Artificial steps or discontinuities can appear in maps and charts if they are built from data based on inconsistent vertical datums. This problem can be particularly acute in coastal areas. On a gently sloping beach, an offset in elevation will change the depiction of the shoreline - it can shift the shoreline and change its position on the map.

Vertical datum transformations have been generated for several U.S. coastal areas (see map above or icons at the bottom of the page). VDatum has been developed and implemented for the region of Tampa Bay, Florida, as part of the NOAA - USGS (U.S. Geological Survey) Bathymetric/Topographic/Shoreline Demonstration Project. NOAA's Marine Sanctuaries Program provided support to use VDatum with appropriate tidal modeling to blend various bathymetric data sets along the coast of Cailfornia. VDatum and NOAA's Tidal Constituent And Residual Interpolation (TCARI) method were applied to transform bathymetric data for a special Real Time Kinematic (RTK)-GPS hydrographic project in Delaware Bay. VDatum has also been implemented to produce a bathymetric elevation surface along the coast of New York and New Jersey for an offshore aggregates project with the USGS and U.S. Army Corps of Engineers. Because vertical referencing is critical for many important applications, a major goal of the National Ocean Service is to develop a national vertical datum transformation database (National VDatum) that extends VDatum coverage to all United States coastal areas.

Vertical datums come in three categories: those based on a gravity potential, called Orthometric Datums; those based on tidally-derived surfaces such as high or low water, called Tidal Datums; and three-dimensional (3-D) datums realized through space-based systems such as the Global Positioning System (GPS).

Topographic maps from the USGS generally have elevations referenced to an orthometric datum, either the North American Vertical Datum 1988 (NAVD 88) or to the older National Geodetic Vertical Datum 1929 (NGVD 29). The NAVD 88 was affirmed as the official vertical datum for the United States (by a notice in the Federal Register, Vol. 58, No. 120 page 34245) on June 24, 1993. Nautical charts have depths referenced to different tidal datums, which can vary from chart to chart. In the United States, mean lower low water (MLLW) is the official NOAA chart datum. To support harbor and river navigation, bridge clearances are referenced to a mean high water (MHW), and not MLLW.

VDatum converts elevation data (heights and soundings) among 28 different vertical datums. In practice, a user will only have to transform between a few datums. In Tampa Bay, the separation of the tidal surfaces and the North American Datum 1983 (NAD 83) (and other 3-D datums) is in excess of 24 meters. The relationship of NAD 83 to NAVD 88 is defined by the GEOID99 height model (Smith and Roman, 2000 (html)). The relationship of NAVD 88 to local mean sea level (LMSL) is obtained by interpolating data at leveled tidal benchmarks, and the tidal datums are obtained from tidal datum modeling, or by spatial interpolation that takes land-water boundaries into account.

The VDatum transformation tool operates in two modes: batch mode and interactive mode. As the names imply, the interactive mode allows the transformation of one elevation at a time, and the batch mode allows the transformation of files of points from one vertical datum to another. The above image of the VDatum GUI shows an example of how to convert a single sounding in feet from the 3-D datum NAD 83(86) to NAVD 88. For batch mode processing, elevation data (soundings and elevations) are loaded into a delimited ASCII x,y,z file with a user definable fourth field (e.g. source_id) and executed. The output format is an ASCII delimited format. Elevation values not converted by VDatum (for locations that fall outside the areas where the NAVD 88-to-LMSL conversion and the tidal datum fields have been computed) are tagged with a null data value.

VDatum is written in Java2, and is distributed in both source code form and in Java bytecode embedded in a Java (JAR) file. The source code and algorithms are open source. Running VDatum requires installation of the Java Runtime Environment (JRE) or installation of a Java development package such as the Java Software Development Kit (JDK). Users who do not have Windows will need to obtain a JRE or JDK for their computer platform.

VDatum v. 1.06 software and documentation are available for the following project areas.
Please click on the appropriate image to download VDatum in a zipped file (.zip). The size of each zipped file is noted in kilobytes below the project area name.

Details about the files and tutorial included in each zipped file available at--> vdatum.pdf.
Sample test data and format for Tampa Bay is available at--> vdatum_input.txt.

Publications

Hess, K. W., 2001:   Generation of Tidal Datum Fields for Tampa Bay and the New York Bight.  U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Silver Spring, Maryland, NOAA Technical Report NOS CS 11, 43 p.

Hess, K. W. and Gill, S. K., 2003:   Puget Sound Tidal Datums by Spatial Interpolation.  Proceedings, Fifth Conference on Coastal Atmospheric and Oceanic Prediction and Processes, American Meteorological Society, Seattle, August 6-8, 2003. Paper 6.1, 108-112.

Hess, K. W., Milbert, D. G., Gill, S. K., Roman, D. R., 2003:    Vertical Datum Transformations for Kinematic GPS Hydrographic Surveys. (pdf)  U.S. Hydrographic Conference, Biloxi, Mississppi, March 24-27, 2003.

Hess, K. W. and White, S. A., 2004:    VDatum for Puget Sound: Generation of the Grid and Population with Tidal Datums and Sea Surface Topography.   U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Silver Spring, Maryland, NOAA Technical Memorandum NOS CS 4, 27 p.

Milbert, Dennis G., 2002:  Documentation for VDatum and VDatum Tutorial:  Vertical Datum Transformation Software, Version 1.06 (pdf) (http://nauticalcharts.noaa.gov/bathytopo/vdatum.htm).

Milbert, D. G. and Hess, K. W., 2001:  Combination of Topography and Bathymetry through Application of Calibrated Vertical Datum Transformations in the Tampa Bay Region. (pdf)   Proceedings of the Second Biennial Coastal GeoTools Conference, Charleston, South Carolina, January 8-11, 2001.

Parker, B., 2002:   The Integration of Bathymetry, Topography, and Shoreline and the Vertical Datum Transformations Behind It.   International Hydrographic Review, Volume 3, Number 3, November 2002.

Parker, B., Hess, K. W., Milbert, D. G., Gill, S., 2003:    A National Vertical Datum Transformation Tool.  Sea Technology, Volume 44, Number 9, pp. 10-15, September, 2003.

Parker, B., Hess, K. W., Milbert, D. G., Gill, S., 2003:    National VDatum - The Implementation of a National Vertical Datum Transformation Database.  (pdf) U.S. Hydrographic Conference, Biloxi, Mississppi, March 24-27, 2003.