The National Geodetic Adjustment (NGA) is a dataset used by the National Geodetic Office to manage geodetic observations. It is used to compute a nationally consistent set of NZGD2000 coordinates, including NZVD2016 heights.
Most geodetic observations held by LINZ including: GNSS, terrestrial observations and 1V levelling, have been combined in to a single dataset. The observations are weighted according to their observation type and accuracy, and the latest version of the deformation model and post earthquake patches are applied.
Using coordinates from the Order 0 PositioNZ CORS sites, the observations are adjusted using least squares in order to determine the coordinates (including heights) of geodetic marks. These coordinates are recorded as at the nominal reference epoch 1st January 2000.
Least squares is a mathematical approach used to determine the best fit of data. LINZ use the least squares package SNAP (Survey Network Adjustment Package) to perform this adjustment.
The resulting coordinates are assessed against accuracy standards, and coordinate orders are assigned. NZVD2016 heights are also computed and assessed against accuracy standards. These coordinates are used to update the data published on the Geodetic Database.
The national geodetic adjustment is re-run periodically, once new geodetic data has been added or after a new version of the national deformation model is added.