The Elements of Geodesy: The Vertical Datum

The position of this vertical survey marker in Louisiana has been upset due to significant subsidence (sinking) of the surrounding area. Click on the image for a larger view.

The vertical datum is a collection of specific points on te 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, round brass plates mark positions in the vertical datum. The traditional method for setting these vertical benchmarks is called differential leveling. This method uses a known elevation at one location 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 benchmarks 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 benchmarks. In 1988, NGVD 29 was adjusted to remove inaccuracies and to correct distortions. The new datum, called the North American Vertical Datum of 1988 (NAVD 88), is the most commonly used vertical datum in the United States today.

Highway 23 is the main hurricane evacuation route for the entire state of Louisiana. Vertical benchmarks used together with the Global Position System have been critical in tracking rates of subsidence in the area and allowing officials to develop emergency evacuation plans. Click on the image for a larger view.

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 high population levels. 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.


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