New Zealand Map Grid (NZMG)

This page contains information on the NZMG, which was used for 1:50,000 topographic mapping in New Zealand until 2001, when it was replaced by the NZTM2000.

The NZMG is the NZGD1949 projection.

This projection was used to produce the existing series of NZMS260 1:50,000 topographic maps (with the exception of the Chatham Islands).

Unlike NZTM2000, NZMG is not based on a geometric projection (transverse Mercator is based on a cylinder). Instead it uses a complex-number polynomial expansion. This has the advantage of exhibiting minimal scale distortion over New Zealand; however it is a projection unique to New Zealand and so can be difficult to use or program into computer software or positioning devices (eg, GPS receivers).

The NZMG is based on the NZGD1949 datum, whereas NZTM2000 is based on NZGD2000. This means that the transformation of coordinates between the two projections is not a straight forward process. Similarly, it is not possible to have NZMG coordinates in terms of NZGD2000 and vice versa. More information about converting coordinates between the NZTM2000 and NZMG projections is available on the coordinate conversion pages.

NZMG is formally defined in Reilly WI (1973) A Conformal Mapping Projection with Minimum Scale Error (attachment below). The key parameters are summarised below:

Name:New Zealand Map Grid
Projection type:Conformal (orthomorphic)
Reference ellipsoid:International 1924
Origin latitude:41° 00' 00" South
Origin longitude:173° 00' 00" East
False Northing:6,023,150 metres North
False Easting:2,510,000 metres East

Due to its complexity, "C" language source code is available to facilitate the incorporation of the projection in the development of software applications.