In New Zealand, there are two main height systems. Ellipsoidal heights are often used with GNSS (global navigation satellite system) measurements. Normal-orthometric heights are more widely used, especially for practical applications such as levelling.
Ellipsoidal heights are convenient to calculate as they use a basic geometric shape to approximate the Earth’s surface. However, they do not account for gravity so are not suitable for determining the flow of water. On the other hand, normal-orthometric heights consider the relationship between the Earth’s surface and its gravity field. This makes them difficult to calculate, but they allow accurate determination of waterflow and are used extensively in engineering and planning.
The term ‘normal-orthometric’ refers to heights measured as the distance from the quasigeoid (a hypothetical reference surface representing a mean sea level) to a point along the curved normal gravity plumbline (a line pointing toward the centre of the Earth due to gravity). Normal-orthometric heights are determined using an approximation of the gravity field (as opposed to actual observations which would be necessary for a ‘normal’ height), and without considering the density of the material beneath the surface (which would be necessary for an ‘orthometric’ height).
The heights published by Toitū Te Whenua in the Geodetic Database and Landonline in terms of New Zealand Vertical Datum 2016 (NZVD2016) and the 13 local vertical datums are all normal-orthometric heights. However, on this website and in many publications, the terms ‘normal-orthometric’ and ‘orthometric’ are used interchangeably.