Last update 2009/10/01
USDOV2009 is a companion product derived from the 2009 gravimetric geoid model, and represents the deflections of the vertical at the surface of the Earth. These quantities are typically a few arc seconds, but can reach an arc minute of departure.
The deflection of the vertical is the departure of a plumb bob's actual pointing from the ellipsoidal normal direction. Deflections are used to relate the orientation of a locally-leveled instrument, such as a theodolite, to a spatial reference system. Important uses are corrections to zenith distance (vertical angle) measurements, and the conversion between astronomic and ellipsoidal azimuths (the Laplace correction).
USDOV2009 is comprised of both north-south (Meridian component,
designated as "Xi") and
east-west components (Prime Vertical component, designated as "Eta").
In the United States, Xi ranges from +41 arcseconds (Bahamas),
to -38 arcseconds (Bermuda), while Eta ranges from +45 arcseconds (Rockies)
to -43 arcseconds (Bermuda).
Currently, USDOV2009 products are available for the Continental U.S.,
Hawaii, Guam and the Northern Mariana Islands, and American Samoa.
North-South Deflections of the Vertical (xi)
East-West Deflections of the Vertical (eta) |
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