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1. Home
2. Education
3. Geodesy
4. The Vertical Datum

**The Vertical Datum** Global Positioning Tutorial  The vertical datum is a collection of specific points on the Earth with  known heights either above or below mean sea level. Near coastal areas,  mean sea level is determined with a tide gauge. In areas far away from  the shore, mean sea level is determined by the shape of the geoid. Similar to the survey markers used to identify known positions in the  horizontal datum, many positions in the vertical datum are marked by steel rods driven  into the ground  with a hinged access cover. Using a technique called differential leveling, a known elevation at one  location is used to determine the elevation at another location. As with  horizontal datums, the advanced technology of GPS has almost completely  replaced this classical technique of vertical measurement. In 1929, the National Geodetic Survey (NGS) compiled all of the  existing vertical bench marks and created the National Geodetic Vertical  Datum of 1929 (NGVD 29). Since then, movements of the Earth's crust  have changed the elevations of many bench marks. In 1988, NGVD 29 was  mathematically adjusted to remove inaccuracies and to correct distortions. This datum, called the North American Vertical Datum of 1988 (NAVD 88), is the most commonly used vertical datum in the United States today. In 2022, scientists at the National Geodetic Survey are scheduled to complete 15-year-long project to update the vertical datum, which will make accurate height measurement better, faster, and cheaper. This project is called the Gravity for the Redefinition of the American Vertical Datum, or GRAV-D for short. When this effort is completed, users will be able to get accurate heights to within about an inch for most locations around the nation.  Brass monument in the midst of a subsidence area.*The position of this vertical survey marker in Louisiana has been upset due to significant subsidence (sinking) of the surrounding area. Similar to the survey markers used to identify positions in the horizontal datum, round brass plates mark positions in the vertical datum. The markers are embedded in concrete or bedrock to maintain their positions and, therefore, the integrity of the geodetic control point.*  One of the main uses of the vertical datum is to measure rates of  subsidence, or land sinking. In Louisiana, for example, large areas of  land are rapidly sinking. This is the result of development, coastal  erosion, and groundwater withdrawals. In many areas, the only way to  escape an incoming hurricane is to follow specific hurricane evacuation  routes. If state and local officials do not have accurate elevation  information about these routes, residents trying to leave during an  emergency might get trapped in fast-rising water. By  referencing the vertical datum, officials can determine the true  elevation and position of the hurricane evacuation routes, as well as  how much they have sunk over past decades. For example, in Plaquemines  Parish, Louisiana, the main hurricane evacuation route, Highway 23, and  the surrounding levees are subsiding by one-quarter to one-half inch  per year.  Brass monument in the midst of a subsidence area.*Highway 23 is the main hurricane evacuation route for the entire state of Louisiana. Vertical bench marks have been critical in tracking the rate of subsidence in the area and allowing officials to develop up to date emergency evacuation plans.*
  Geodesy **Lessons** Welcome What is Geodesy? The History of Geodesy The Geoid What is a Datum? The Horizontal Datum The Vertical Datum Gravity The National Spatial Reference System The Global Positioning System CORS and GIS References
  More **Information** Roadmap to Resources Subject Review Is the Earth round? (Ocean Facts)  Geodesy Education Heard of geodesy? These videos from NOAA's National Geodetic Survey offer a deep dive into the science of knowing where you – and everything else is in the world!
  The National Science Teaching Association (NSTA) includes this resource in its database. NSTA provides educators and students access to Web-based, educationally appropriate science content that has been formally evaluated by master teachers.
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  **Last updated:** 02/26/21 **Author:** NOAA How to cite this article Contact Us


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