Like the peel of an orange, it is not possible to lay the surface of the Earth flat without breaking or distorting it in some way. Map makers use projections to systematically manage these distortions. There are many different types of projection that can be used. The most common are related to different geometric shapes such as cylinders, cones and planes. In New Zealand, most new maps are produced using the Transverse Mercator projection, which is based on a cylinder that lies in a north-south direction. The official projection for New Zealand topographic mapping is called the New Zealand Transverse Mercator 2000 (NZTM2000). Until very recently in New Zealand many topographic maps were produced using the New Zealand Map Grid (NZMG) projection. This has three major differences from NZTM2000: Projections are also used to represent digital geodetic data, GPS receivers often print out projected coordinates as well. An easy way to tell if a coordinate is in terms of a projection is to check its units. If it refers to metres north or east, then it is probably projected, if it is degrees north or south, then it will be a datum coordinate. When using maps or coordinates, it is critical to know what projection and/or datum is being used. When using coordinates or heights from different sources it is important to ensure that they are in terms of the same datum and/or projection. If coordinates or heights are mixed, this can introduce errors (potentially of hundreds of metres) in the positions resulting from them. A description of the different coordinate conversions and height conversions is available on this site. It is also possible to convert coordinates online or to download a software application for offline use (Microsoft Windows only). Source.