Featherstone & Morgan: Validation of the AUSGeoid98 model in WA 
Given that the assumptions about the plumbline and 
quasigeoid have no appreciable effect, timing errors in 
the original (before 1966) astronomical longitudinal 
observations (c/. Kearsley 1976) are a more plausible 
cause of the observed differences. This is implied in Table 
lb, where - even after outlier detection - the longitudinal 
(east-west) deviation discrepancies are larger than the 
latitudinal (north-south) differences. 
Summary and Conclusion 
In this short paper, we have used a recently released 
set of historic (pre-1966) astronomically observed vertical 
deviations to independently verify the AUSGeoid98- 
computcd vertical deviations across Western Australia. 
Our results agree with earlier studies (e.g., Jekeli 1999; 
Featherstone 2006), showing that vertical deviations are a 
useful independent validation of a gravimetric 
quasigeoid model, but the vintage of the astrogeodetic 
data, particularly in longitude/time, is a limiting factor. 
We have shown that the AUSGeoid98-computed 
vertical deviations are generally of sufficient precision 
(one arc-second standard deviation) to support the 
reduction of terrestrial-geodetic survey data in Western 
Australia. A new Australia-wide gravimetric quasigeoid 
model is currently in preparation, awaiting the release of 
new datasets from dedicated satellite gravimetry, which 
should improve the situation yet further. 
Acknowledgements: Wc would like to thank the three anonymous 
reviewers and editor for their very constructive critiques of this 
manuscript in a very short timeframe. WEF would like to thank the 
Australian Research Council (ARC) for funding research on the 
Australian quasigeoid and AHD through grants A4933I318, A39938040, 
DP02n827 and my Australian Professorial Fellowship through grant 
DP0663020. Thanks also go to Jim Steed (Ret.) of Geoscience Australia 
and Dr Simon Holmes of SGT Inc. for providing some of the astrogeodetic 
vertical deviations. The figures in this manuscript were produced with 
the public-domain Generic Mapping Tools (Wessel and Smith, 1998; http:/ 
/gmt.soest.hawaii.edu/). This is The Institute for Geoscience Research 
(TIGeR) publication number 61. 
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