**Warning:** This raster is a grid of a floating-point values; not a surface. To derive an accurate height transformation value, this raster grid must be downloaded in terms of NZGD2000 and then converted into a surface using bilinear interpolation.

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**Introduction**

This dataset provides a 1 arc minute raster image of the free-air gravity anomalies, which have been downward continued to the ground surface (McCubbine et al, 2017).

**Description**

Gravity anomalies are differences between measured gravity (from the airborne gravity dataset) and an ellipsoidal model of the Earth’s gravity field (GRS80). Gravity anomalies correspond to un-modelled density variations within the Earth’s crust and upper mantle. They are used to investigate concealed geological structures and for quasigeoid modelling.

These free-air anomalies show values which include gravitation impact of the topography.

The national airborne gravity dataset is comprised of more than 50,000 linear km of flight observations, covering the three main islands of New Zealand and up to 10km offshore.

As the airborne gravity dataset was measured at flight altitude, the observations have been reduced to the ground surface (a process known as downward continuation).

The national airborne gravity dataset was collected as a joint project between Toitū Te Whenua Land Information New Zealand (LINZ), GNS Science (GNS) and Victoria University of Wellington (VUW). The airborne survey was completed in a total of eight months, over two campaigns: August – October 2013, and February – June 2014.

**Users may also be interested other layers created for Bouguer anomalies at ground surface and the along track observations from the gravity flight lines at flight elevation** [NZ Airborne Gravity Bouguer Anomalies at Ground Surface (2013-2014)]( https://data.linz.govt.nz/layer/3530 ) and [NZ Airborne Gravity Flight Lines at Elevation (2013-2014)]( https://data.linz.govt.nz/layer/ 3531).

McCubbine, J. Stagpoole, V. Caratori-Tontini, F. Amos, M. Smith, E. and Winefield, R. (2017). Gravity anomaly grids for the New Zealand region. Manuscript submitted for publication New Zealand Journal of Geology and Geophysics.