Amos & WEF 2003 "Preparation of a new geoid model"

Source: Amos & WEF (2003) “Preparation of a new ....” NZ Surveyor

1. The paper described that free-air gravity anomalies were computed;

• from IGSN71-corrected gravity observations
• by subtracting the value of normal gravity at the geocentric observation latitude,
• then adding the second-order free air correction and atmospheric correction for the for the observation elevation (above local MSL)

2. A second-order free air correction is a more accurate representation of the vertical gradient of gravity for an ellipsoidal Earth because it takes into account the variation of normal gravity with latitude as well as higher order terms in height.

3. Why do atmospheric correction?

i) AC accounts for the mass-inconsistency between the GRS80 normal ellipsoid and gravity observed        on the Earth surface, as well as
ii) the gravitational attraction of the atmosphere above the gravity observation point,
iii)  AC also needed to make the terrestrial gravity anomalies consistent with those derived from the GGM

4. How to apply AC? Relevant formulas are given in

i) WEF (1995)
ii) WEF et al (1997)
iii)  Further work on AC is described by Sjoberg (2000)

Torge, 1989 – Gravimetry, Gruyter .... mentioned that the Potsdam gravity datum contains an error (... therefore IGSN71 global gravity datum should be used instead)

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