A vertical datum is a reference system that defines a surface which provides a ‘zero point’ from which heights are measured. Just as latitude and longitude on the Earth's surface are measured using geodetic datums, elevations and depths are measured using vertical datums. Having a consistent vertical datum is vital for managing water resources, assessing hazards, and carrying out large-scale engineering.
The different types of vertical datum which are used in New Zealand are either normal-orthometric or ellipsoidal.
Normal-orthometric datums define a geoid which is dependent on the Earth’s gravity field. Examples include New Zealand Vertical Datum 2016 (NZVD2016) and the 13 official local vertical datums.
New Zealand Vertical Datum 2016 (NZVD2016)
Ellipsoidal datums are in reference to an ellipsoid which approximates the Earth’s shape. An example is New Zealand Geodetic Datum 2000 (NZGD2000).
New Zealand Geodetic Datum 2000 (NZGD2000)
Heights can be converted between datums by using a transformation surface such as the New Zealand Quasigeoid 2016 (NZGeoid16) or a vertical datum relationship grid. The formula used in these transformations are provided on the page Equations for vertical datum transformations.
New Zealand Quasigeoid 2016 (NZGeoid16)
Vertical datum relationship grids
Equations for vertical datum transformations
The general relationship between the vertical datums used in New Zealand is shown in the figure below.
Diagram showing the relationship between different height surfaces
Converting data between vertical datums our online coordinate converter or the vertical datum conversion form. Both give examples of the expected input data depending on the options chosen.
To convert larger datasets such as point clouds, more technical instructions are provided in the Medium post: Reprojecting point clouds to NZVD2016
Vertical datum conversion form
Note that when using relationship grids in software such as GIS, the raster images provided on the LINZ Data Service are not a transformation surface. A surface must be created using a bi-linear interpolation method to determine the correct offset values.