Airborne Gravity Across New Zealand - for an Improved Vertical Datum

Abstract:

J. McCubbine¹, Euan Smith¹, Matt Amos², Rachelle Winefield² & Fabio Caratori Tontini³

1 Victoria University of Wellington, Kelburn Parade, Wellington.

2Land Information New Zealand, National Geodetic Office, Wellington.

3GNS Science, Lower Hutt.

Land Information New Zealand in collaboration with GNS Science and Victoria University has completed the first

national airborne gravity survey of New Zealand. The aim of the programme was to determine gravity anomalies at

a 10 kilometre wavelength and thereby compute a national geoid with at least 3 centimetre accuracy.

The airborne gravity data consist of a uniform set of measurements that cover the whole of New Zealand, including

the shallow coastal areas and rough topography that have previously been extremely difficult to survey. Over

50,000 line-kilometres of surveying were completed in two campaigns during August - October 2013 and February –

June 2014.

After the removal of erroneous data and flight line biases, using novel methods, a repeatability of approximately 2

mGal has been achieved and agreement with terrestrial gravity measurements of 3.5 mGal after downward

continuation along two calibration lines. Overall the airborne data have a cross over error of around 5.8 mGal and a

difference from existing terrestrial data with a standard deviation of around 8 mGal and zero mean.

A method to efficiently compute terrain corrections using a modified version Hammer’s formula to approximate a

prism has been employed and a combined gravity anomaly grid of the airborne and existing terrestrial data has been

calculated by least squares collocation. This grid is then used to determine the new gravimetric geoid which is

compared to GNSS levelling data.