Software Documentation

The VDatum transformation software (Milbert, 2002 (pdf)) encodes a four-step traversal path along a minimal spanning tree whose nodes represent the vertical datums categorized as tidal datums, orthometric datums, and ellipsoidal (3-D) datums. Transformations between the three main categories rely on datum conversions to the primary datum of each category:  Local Mean Sea Level for the tidal datums, North American Vertical Datum of 1988 (NAVD 88) for the orthometric datums, and North American Datum of 1983 (1986) for the ellipsoidal datums.

Currently, 28 different vertical datums are incorporated into the VDatum software. Conversions between the ellipsoidal datums apply the seven-parameter Helmert transformation that ascribes the 3-D distance, rotation, and scale changes. To account for coastal land-water boundaries, the tidal datums within VDatum are computed from calibrated numerical hydrodynamic models, or by spatially interpolating water levels computed from tide gauge measurements (Hess et al. 1999; Hess, 2002, 2003).

Sea surface topography, which relates the NAVD 88 orthometric heights to Local Mean Sea Level, is regionally interpolated from water depth measurements taken at leveled tidal benchmarks in each area. Based upon geopotential fields from the Earth Gravity Model of 1996, the GEOID99 height model (html) relates the NAVD 88 orthometric heights with the North American Datum of 1983 (1986) ellipsoid heights (Smith and Roman, 2000; Smith and Roman, 2001 (html)). Future software upgrade to the GEOID03 height model will substantially increase spatial resolution and reduce interpolation errors (Roman et al. 2004 (pdf)).

The VDatum software operates in interactive mode or in batch mode. As the names imply, the interactive mode allows the transformation of one elevation at a time, while the batch mode allows the transformation of files of points from one vertical datum to another.

The above image of the VDatum GUI illustrates how to convert a single sounding in feet from the 3-D ellipsoidal datum NAD 83(86) to NAVD 88. For batch mode processing, elevation data (soundings and heights) 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 where the tidal datum fields have been computed) are tagged with a null data value.

VDatum is written in Java2, and is distributed in open source code form and in Java bytecode embedded in a Java (JAR) file. 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 the Windows PC operating system will need to obtain a JRE or JDK for their computer platform.

VDatum version 1.06 general software documentation and a datum tutorial are available at--> VDatum106 (pdf). Specific documentation accompanies the VDatum software for each project area.

Disclaimer: Although many of the vertical datum transformations between the North American Vertical Datum of 1988 (NAVD 88) and mean sea level, and between mean sea level and the other tidal datums, are based on tidal values from the present National Tidal Datum Epoch (NTDE, 1983 to 2001), some are based on data from older tidal epochs. NOS is in the process of updating the data in VDatum to conform to the latest NTDE. In the meantime, care should be used when applying these transformations.