A projection is used to convert points on a 3-dimensional curved surface of the Earth onto a 2-dimensional flat surface. Unlike geodetic datums, which maintain the accuracy of coordinates, projections distort the data so that it can be shown on maps and visualisations on a flat piece of paper or a screen.
Because projections represent a curved surface on a flat plane, they need to distort the data. Different projection methods have their own strengths and weaknesses, with some preserving area, while others maintain angles or distances. This means that particular projections will better suit different applications and regions.
Most projections are based on cylinders, cones or planes. The most common projection used in New Zealand is the Transverse Mercator projection, which is based on a cylinder that lies on its side and is aligned with a line of longitude (the origin or central meridian).
Transverse Mercator projection
Projections commonly use cartesian coordinates such as easting and northing, which correspond to the x and y axes on a map. These coordinates are measured in meters or other linear units depending on the specific projection. For example:
- Easting (x-axis): 1,500,000 m
- Northing (y-axis): 5,175,000 m
Many projections use a false northing and easting, so the origin is a large number instead of 0m east, 0m north. For example, New Zealand Transverse Mercator (NZTM2000) uses the origin 10,000,000 m north, 1,600,000 m east. This ensures that all coordinates on the map have a positive value, which simplifies calculations. It also means that coordinates using NZTM2000 have a distinctive range, so users can identify the projection that a set of coordinates relate to.
The official projections used in New Zealand are in terms of the New Zealand Geodetic Datum 2000 (NZGD2000). These are:
- New Zealand Transverse Mercator 2000 (NZTM2000)
- NZGD2000 meridional circuits
- New Zealand offshore island projections
- New Zealand Continental Shelf Lambert Conformal 2000 (NZCS2000)
- Ross Sea Region projections
Four historic projections were commonly used in New Zealand: