Heights are measured above (or below) a reference surface called a datum.
There are two main types of height used in New Zealand: normal-orthometric heights and ellipsoidal heights.
Normal-orthometric heights are measured above a surface that approximates the mean level of the sea as a reference surface. Examples of normal-orthometric height datums are NZVD2016 and 13 local vertical datums.
Ellipsoidal heights refer to geometric approximation of the Earth called an ellipsoid. In New Zealand ellipsoidal heights can differ from sea level by over 35 metres. An example of an ellipsoidal datum is NZGD2000. GNSS (GPS) receivers measure ellipsoidal heights.
Heights can be converted between normal-orthometric and ellipsoidal systems by using a geoid model such as NZGeoid2016.
In New Zealand there are three main vertical datums that are commonly used:
- New Zealand Vertical Datum 2016 (NZVD2016)
- Local mean sea level datums
- New Zealand Geodetic Datum 2000 ellipsoidal heights.