Tuesday 23 August 2005

G2

PT0009
A unified national height system for New Zealand
Amos, Matthew¹, Featherstone, Will²
1 Land Information New Zealand, Wellington, New Zealand
2 Western Australian Centre for Geodesy, Curtin University of Technology, Perth, Western Australia
Author email: mamos@linz.govt.nz
New Zealand does not have a single national vertical datum; instead 13 disparate vertical datums are used, each of which is connected to a separate tide gauge. The datums are known to be offset from each other, but the magnitude of the offset has been difficult to quantify due to the absence of direct levelling connections between many of them. To enable the unification of these 13 local datums, Land Information New Zealand has approved the implementation of a new national vertical datum that is independent of local mean sea level. This paper presents the concept of a new national vertical datum for New Zealand. It will be based on ellipsoidal heights, and the relationship between the 13 existing vertical datums relative to the ellipsoid has been established using a high-precision regional gravimetric geoid model. The potential for vertical datum unification using a gravimetric geoid is well known. However, the effect of reducing gravity data to offset vertical datum surfaces seems to be rarely appreciated. The approach used here both acknowledges this bias and applies a correction to the gravity anomalies used for geoid computation to enable a more rigorous unification solution. This results in a marginally better fit of the computed geoid to GPS-levelling observations.

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