Land Information New Zealand completed the first national airborne gravity survey of New Zealand in collaboration with GNS Science and Victoria University of Wellington. Over 50,000 line-kilometers of data were collected to determine gravity anomalies with 10km spatial resolution. This data was combined with existing terrestrial and satellite data using least squares collocation to produce a new gravimetric quasigeoid model. Validation showed the new model has an accuracy of 3.9cm standard deviation and 4.86cm root mean squared error, an improvement of around 1.5cm over the previous NZGeoid2009 model.

1. Airborne gravity across New Zealand - for an improved vertical datum.
By Jack McCubbine1
, Dr. Euan Smith1
, Dr. Matt Amos2
, Rachelle Winefield2
& Dr. Fabio Caratori Tontini3
Contact: Jack.c.mccubbine@gmail.com
Introduction
Land Information New Zealand in collaboration with GNS Science and Victoria University of Wellington has completed
the first national airborne gravity survey of New Zealand. The aim of the programme was to determine gravity
anomalies with a 10 kilometre spatial resolution, combine the airborne data with existing gravity data and compute a
new gravimetric quasigeoid which is more accurate than the current official national quasigeoid NZGeoid2009.
References
Amos, M., 2007, Quasigeoid Modelling in New Zealand to Unify Multiple Local Vertical Datums. PhD thesis, Curtin University of Technology, Perth, Aus-
tralia.
Featherstone, W., Evans, J. and Olliver, J.G, 1998, A Meissl-modified Vanicek and Kleusberg kernel to reduce the truncation error in gravimetric geoid
computations. Journal of Geodesy, Vol. 72, No. 3.
Claessens, S., Hirt C., Featherstone, W., Kirby, J. 2009, Computation of a new gravimetric quasigeoid model for New Zealand. Western Australian Centre
for Geodesy Curtin University of Technology.
The Airborne Gravity Survey
Over 50,000 line-kilometres of airborne gravity surveying
were completed in two campaigns during August - October
2013 and February – June 2014. The acquired airborne
gravity data are a spatially uniform set of measurements
which cover the whole of New Zealand, and includes
shallow coastal areas and rough topography that have
previously been extremely difficult to survey.
The plane used for the survey was a Piper Chieftain and the
gravimeter was a Lacoste and Romberg model ‘S’.
Two calibration lines were flown 5 times and gave
a measure of 2.5 mGal standard deviation for the
repeatability of the data. These lines are marked
in red on the map.
The difference between data along different flight
lines were assessed at intersection points. This
method of evaluating the accuracy of the data was
influenced predominantly by the necessary use of
an along track filter, but also the gravitational
effect of the terrain and the relative flight line
elevations. After adjusting for these effects the
intersection difference had a standard deviation of
5.4 mGal.
The Combined Gravity Anomaly Grid
The airborne gravity data have been augmented with existing terrestrial gravity data (approx.
40,000 measurements), the latest Sandwell and Smith marine gravity anomaly (v23.1) and
shipborne gravity data in the region of 25(S) to 60 (S) and 160(E) to 190(E). The data were
corrected for the gravitational effect of the topography and then combined using least squares
collocation, gridding at the surface of the Earth.
The effect of the topography was restored using 1 arc-minute block averaged orthometric heights
and a reverse Bouguer slab correction to obtain a gridded Faye anomaly. These data provide a
gridded regional gravity anomaly with unprecedented uniformity and internal consistency for
quasigeoid evaluation.
New Gravimetric Quasigeoid Computation
All of the most recent (from 2007 onwards) global gravity models from http://icgem.gfz-potsdam.de/ICGEM/modelstab.html
have been compared to 1422 levelling /GPS derived height anomalies. The Eigen-6C4 model minus these data had the
smallest RMS and standard deviation of 5.3cm and 4.5cm respectively.
A new gravimetric quasigeoid has been computed from the gridded gravity data with Stokesian integration and the
remove-compute-restore technique using Eigen-6C4 for the reference signal. The Featherstone et al (1998) modified Stokes
kernel was used and a modification degree of 280 and spherical cap of 1.5° has been chosen since it gave a restored
quasigeoid with the best fit to the levelling data.
Figure 3: The Bouguer gravity anomaly from the airborne
gravity survey. A 64m resolution digital elevation model has
been used for the terrain corrections out to 120 km from
the observation point. The scale is in mGal *-100 150+.
Figure 4: Gridded Faye anomaly, determined from the combined airborne,
terrestrial, satellite altimetry and shipborne gravity data. At the surface of the earth
using least squares collocation with a logarithmic covariance model.
The scale is in mGal *-120 150+.
New Gravimetric Quasi Geoid Accuracy Assessment
A new gravimetric quasigeoid has been calculated with an estimated accuracy of 3.9 cm standard deviation and 4.86 cm root
mean squared on comparison to the levelling data nation wide. This is around 1.5 cm better than the NZGeoid2009. The
levelling data are relative to one of 13 local mean sea level estimates, a local vertical datum mean offset has been computed
and removed from the differences.
The accuracy of the new quasigeoid has also been evaluated in New Zealand’s main urban areas, Auckland, Wellington and
Christchurch. The difference between the quasigeoid and levelling derived geoid heights in these regions have standard
deviations of 3.46, 3.91 and 1.9 cm and RMS’s of 3.52, 4.30 and 4.32 cm respectively.
Figure 2: Flights along calibration line in red and
mean in green, North Island (Top) South Island
(Bottom).
Figure 1: Survey flight lines (Top), Plane used for the survey
Piper Chieftain ZKRTD (Bottom Left), Gravimeter used for the
survey, Lacoste and Romberg model ‘S’ (Bottom Right).
Figure 5: The Eigen-6C4 global gravity model quasigeoid
undulations over the region 25(S) to 60(S) and 160 (E) to 190
(E). The scale is in metres *–45 60+.
Figure 6: The residual quasigeoid undulations determined
by the remove-compute-restore technique over the region
25 (S) to 60(S) and 160 (E) to 190 (E). The scale is in metres
*-0.25 0.25+.
Figure 7: The derived height anomalies of 1422
coupled leveling & GPS observation sites across
New Zealand. The scale is in metres *-5 40+.
Figure 8: A new gravimetric quasigeoid
minus the leveling derived height anomalies.
The scale is in metres *-0.25 0.25+.
Figure 9: The current official national
quasigeoid (NZGeoid2009) minus the leveling
derived height anomalies. The scale is in
metres *-0.25 0.25+.