Which Coordinate System to use in New Zealand?
By: Toitū Te Whenua Linz
Tags: Projection Mapping, New Zealand, Geographic Information, Coordinate Systems, Geospatial

It can be difficult to be sure what coordinate system to use. This post will provide guidance to determine the best coordinate system to use in New Zealand depending on different situations.

Types of Coordinate System

We will look at three types of coordinate systems: geographic, geometric and vertical.

Geographic (or Geodetic) Systems

These relate the physical location of a point on a curved surface with a coordinate in terms of latitude, longitude, and height; as illustrated by the following diagram.

In New Zealand we have the choice between the New Zealand Geodetic Datum 2000 (NZGD 2000) which is the official datum for New Zealand, or the World Geodetic System 1984 (WGS 84) which, as the name suggests, is a global geographic system. They are often regarded as the same, but NZGD 2000 provides sub-metre accuracy by accounting for the local deformation in New Zealand. WGS 84 by contrast is a global datum which means that coordinates change over time for local objects which are fixed in the ground.

This is because the tectonic plates on which New Zealand sits are constantly moving, albeit reasonably slowly. In New Zealand this movement is about 5cm per year. This continuous ground movement means that even in the absence of earthquakes and other localised land movements, WGS 84 coordinates are constantly changing. These are often referred to as dynamic or kinematic coordinates. It is important that coordinates in terms of WGS 84 have a time associated with them, especially where the best levels of accuracy are required.

Projected (or Geometric) Systems

Projected systems are a type of cartesian system that maps the earth on a flat surface with coordinates recorded as x/y values as shown in the following diagram.

It is not possible to represent geographic coordinates on a flat surface without introducing some sort of distortion. This can be visualised using the peel of an orange: it is not possible to lay the entire peel flat without breaking it in some way.

A projection is used to manage these distortions in a predictable way. Projections are chosen to maintain certain characteristics and to minimise the impact of the distortion. Different projections can preserve direction, shape, area, distance and shortest route, however, no projection can preserve all of these.

An additional benefit of projections is that they have units of metres, which lets users measure meaningful distances directly from a map.

The two projections commonly used in New Zealand are New Zealand Transverse Mercator 2000 (NZTM 2000) and Web Mercator. NZTM 2000 is the official projection for New Zealand topographic mapping. Web Mercator is used worldwide and the de facto coordinate system for web mapping applications such as Google Maps, Mapbox, and Bing Maps.

Vertical Coordinate Systems

Vertical coordinate systems record heights measured above (or below) a reference surface (datum).

In New Zealand, there are three main vertical datums that are commonly used: the New Zealand Vertical Datum 2016 (NZVD2016), local mean sea level datums (LVD) and NZGD 2000 ellipsoidal heights.