USDOV2009

North-South Deflections of the Vertical (xi)

East-West Deflections of the Vertical (eta)

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.