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VDatum is a software tool used to transform coastal elevations from one vertical datum
into another (Milbert, 2002; Milbert and Hess, 2001).
Such transformations are necessary when coastal geosptial 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.
VDatum was first developed and implemented for the region of Tampa Bay, Florida,
as part of the joint NOAA-USGS Bathymetric/Topographic/Shoreline Demonstration Project.
Because vertical referencing is critical for many important applications,
a major goal of NOS is to develop a
National VDatum
database that extends coverage to all U.S. 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, 2001). 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, geographic boundaries, and documentation
for Tampa Bay are available for download as a zipped (.zip) file -->
VDatum06_FLtampab01.
General software documentation and a datum tutorial
included in the zipped file are available at --> VDatum106 (pdf).
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