North-South Deflections of the Vertical (xi)
East-West Deflections of the Vertical (eta)
DEFLEC12A is a companion product derived from the 2012A hybrid geoid model (GEOID12A), and represents the deflections of the vertical (DOV's) at the surface of the Earth. Hence, these DOV's are also in the NAD 83 (2011.0) Reference Frame. 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).
DEFLEC12A is comprised of both north-south (Meridian component, designated as "Xi") and east-west components (Prime Vertical component, designated as "Eta"). In DEFLEC12A, Xi ranges from +58 arcseconds (Bahamas), to -54 arcseconds (Bermuda), while Eta ranges from +58 arcseconds (Rockies) to -54 arcseconds (Bermuda).
