Declination around New Zealand
Declination is the difference in horizontal angle between magnetic North and true North. This angle varies with time.
Geomagnetic models show the declination around New Zealand. These models include an estimate of how the field is changing with time. Every 5 years a new model is calculated using the latest information (including international geomagnetic models and New Zealand magnetic data).
Actual magnetic field at a point may not agree with these models because:
1.They do not attempt to model the short-wavelength variations (or anomalies), caused by magnetised rocks in the earth's crust.
There are certain areas of New Zealand where there are substantial magnetic variations due to magnetised rocks some kilometres deep, including a region stretching from Eastern Southland through to Westland, and the so-called Mineral Belt of Nelson, which has a northern extension that runs off-shore up the west coast of the North Island. There are more localised magnetic anomalies around Mt Tapuaenuku in Marlborough, and near East Cape.
Most volcanic rocks are magnetised, so places where volcanic rocks are found at or near the surface often have magnetic anomalies. This includes the areas of volcanoes in Whangarei and Auckland, the Taupo Volcanic Zone, Taranaki (including the ironsand beaches which are derived from volcanic rocks) and the old volcanoes of Banks Peninsula and the Otago Peninsula. In any of these areas, one might find a magnetic compass pointing in a different direction to that expected from the large-scale model.
2. As well as the internally produced magnetic field, the earth has a varying magnetic field that results from its interaction with particles and fields coming from the sun.
Large-scale maps of New Zealand (including the 1: 50000 scale Topo50 and 1: 250000 scale Topo250 series) now use the New Zealand Transverse Mercator 2000 (NZTM) projection. In this projection Grid North differs from True North except at a longitude of 173° E, the mid-line of the projection. The difference is called the convergence. Figure 2 and Figure 3 show the difference between Magnetic North and Grid North in the North Island and South Island respectively for these maps.