DEFLEC99

First created 10/01/1999. DAS.

Last updated 1/29/2001. DAS.

DEFLEC99 is a companion product derived from GEOID99 representing the deflections of the vertical at the surface of the Earth. The deflection of the vertical is the departure of a plumb bob's true pointing from the ellipsoidal normal direction. These quantities are typically a few arc seconds, but can reach an arc minute of departure.

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).

DEFLEC99 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). DEFLEC99 products are also available for Alaska, Hawaii, and Puerto Rico & the U.S. Virgin Islands.

North-South Deflections of the Vertical (xi)

link to a larger image of that shown on this page

East-West Deflections of the Vertical (eta)

link to a larger image of that shown on this page

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