A vertical datum is a reference system (or set of rules) that defines a surface. This surface becomes the ‘zero point’ that heights or elevations are measured above or below. Just as latitude and longitude on the Earth's surface are measured using geodetic datums, elevations and depths are measured using vertical datums. A consistent vertical datum is vital for managing water resources, assessing hazards and carrying out large-scale engineering.
Several different types of local vertical datum are used in New Zealand, they all use normal-orthometric heights. The term ‘normal-orthometric’ (sometimes just called orthometric) represents the vertical distance between a specific point on the Earth's surface and a reference geoid or quasi-geoid surface. Orthometric heights consider variations in the Earth's gravity field, providing accurate measurements which can predict water flow. This is required for most surveying or engineering applications.
There are 3 general types of vertical datum:
- Geoid based heights, such as New Zealand Vertical Datum 2016 (NZVD2016)
The reference surface is a geoid or quasi-geoid, a shape which represents mean sea level by approximating the variations in Earth’s gravity. - Levelling based height, such as the local vertical datums (for example, Auckland Vertical Datum 1949, Moturiki 1953 Vertical Datum). The reference point is an observation at a reference tide gauge at a specific time, and propagated across a network through series of levelling runs.
- Sea surface heights, such as Chart Datum (as used in charting). The reference point is observed sea surface and tidal measurements.
Several different types of local vertical datum are used in New Zealand, they all use normal-orthometric heights. The term ‘normal-orthometric’ (sometimes just called orthometric) represents the vertical distance between a specific point on the Earth's surface and a reference geoid or quasi-geoid surface. Orthometric heights consider variations in the Earth's gravity field, providing accurate measurements which can predict water flow. This is required for most surveying or engineering applications.
Geodetic datum heights (or ellipsoidal heights) may also be used in New Zealand, particularly for GNSS (global navigation satellite system) or GPS (Global Positioning System) applications. However, these heights are not gravity based, and are instead referenced to a geometric approximation of the Earth’s surface. This means that while they may be useful in monitoring height change over time, they should not be used to predict water flow.
Heights can be converted between normal-orthometric systems and to or from ellipsoidal systems by applying corrections from a geoid model or the appropriate relationship grids.
Converting between NZVD2016, NZGD2000 and local vertical datums
Find out how to use the online coordinate converter for height transformations
Toitū Te Whenua is creating relationship grids that will allow transformation between sea surface heights and NZVD2016. This project is called Joining Land and Sea.
The official vertical datum used in New Zealand is New Zealand Vertical Datum 2016 (NZVD2016)