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* Tampa Bay Shoreline Project
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VDatum Transformation Tool (New Version 1.05)

For more info contact: Dennis Milbert Ph.D, Chief Geodesist, National Geodetic Survey, NOAA

This document is part of the "Tampa Bay Bathy/Topo/Shoreline Demonstration Project" - if you are involved in hydrographic and/or bathymetric mapping and working with DEM and topopgraphic data we encourage you to learn more by visiting the Tampa Bay Shoreline project website
VDatum 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 as a "coordinate system" of geo-spatial data. Artificial steps or discontinuties can appear in maps and charts if they are built from data based on inconsistent datums. This problem can be particularly acute in coastal areas. For example, on a gently sloping beach, an offset in elevation will change the depiction of the shoreline- it can shift the shoreline and change its slope on the map.

VDatum has been developed and implemented for the region of Tampa Bay, Florida, as part of the joint NOAA/USGS Bathymetric-Topographic Demonstration Project. The source code and algorithms are based on an open code. And, if this project is successful, the vertical datum transformation methodologies could conceivably be incorporated into various commercial Geographic Information System (GIS) packages.


vDatum GUI


Vertical datums have traditionally come in two categories: those based on a form of Mean Sea Level (MSL), called Orthometric Datums, and those based on tidally-derived surfaces of high or low water, called Tidal Datums. In addition, there is a distinct new third category, consisting of 3-dimensional datums realized through space-based systems such as Global Positioning System (GPS), referred to as 3-D Datums.

Topographic maps from the USGS generally have elevations referenced to an orthometric datums, either the North American Datum Vertical Datum 1988 (NAVD 88) or to the older North American 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 surfaces, which can vary from chart to chart. In the United States the mean lower low water (MLLW) is the typical low water reference surfaces. To support harbor and river navigation, bridge clearances are referenced to a mean high water (MHW), and not MLLW.

GEOID99 height model (Smith and Roman, 2000). The relationship of NAVD 88 to local mean sea level (LMSL) is calibrated from tide model comparisons with leveled tidal benchmarks, and is assumed to be a constant of 0.163 meters in Tampa Bay.

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 transformation of files of points from one vertical datum to another. The above example shows how to convert a single sounding (sounding extracted from DEM) via interactive mode where sounding is being converted from the ellipsoid 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 an optional user definable fourth field (e.g. source_id) and executed. The output format is an ASCII delimited format. Elevation values not converted by VDatum (elevation values that fall outside the range of the hydrodynamic model) are tagged with a -9999 data value.

VDatum is written in Java2, and is distributed in both source code form and in Java bytecode from embedded in a Java (JAR) file. 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). A copy of the JRE for Microsoft Windows is included with VDatum. Users who do not have Windows will need to obtain a JRE or JDK for their computer platform.
Click to enlarge
A merged Topo/Bathy GRID of Tampa Bay was produced at a 1-arc-second and one-third arc-second resolution. High resolution grids were created from the exact same source data as the 1-arc-second grids.
About the Tampa Bay Bathy/Topo/Shoreline Project

This is intended to be the first of several joint demonstration projects in which NOAA's National Ocean Service (NOS) and the USGS's National Mapping Division will blend their bathymetric and topographic data sets into a digital elevation model (DEM). In each case, the NOAA and USGS data will be supplemented with recent high-resolution third-party data, which will be incorporated into the DEM, making use of a newly developed datum transformation tool and quality standards developed in this project. This is the first step toward the development of a mutually agreed upon "national shoreline". For NOAA and USGS it represents the beginning of a new way of doing business with each other that will reduce duplication of effort and better meet the needs of state and county agencies. The applications benefitting from the bathy/topo DEM include: improved hurricane evacuation plans (based on improved storm surge modeling); improved and consistent geospatial data for county planners; better located habitat restoration projects; and detailed electronic nautical charts, to name just a few.

Note: The current version of V-Datum transformation tool that can be downloaded from the web site is for Tampa Bay region only. NOAA and USGS have partnered on other projects in SW Louisiana, NY/NJ Bight and Central California. They are hoping to expand V-DAtum from a regional model (e.g. Tampa Bay) to one that covers the EEZ for the US.

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Are you involved in a similar project and interested in sharing details with The GeoCommunity? If so, we'd like to hear from you. Please send details to Glenn at editor@geocomm.com

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