Local Mean Sea Level Datums

Before the introduction of NZVD2009 in September 2009 heights in New Zealand were referred to one of 13 local mean sea level (MSL) vertical datums. The datums are listed in the table below, their locations and broad extents are shown in the image.

These local datums were been established by determining MSL at a tide-gauge and then transferring this level by precise levelling to benchmarks in the surrounding hinterland. To account for changes in the Earth's gravity field an approximate orthometric correction was also applied to the levelling observations to give normal-orthometric heights in relation to MSL at the tide-gauge origin of that datum.

Sea level is known to vary around the coast of New Zealand. This means that the level of MSL determined each datum's tide-gauge will be different and that offsets will occur between adjacent datums. It is therefore important that the vertical datum of a height is checked before it is used. Also, in most cases the level of MSL for the vertical datums was determined over 50 years ago and has not been updated since then (due to the effect of sea level rise for instance). See tidal level information for surveyors for information about relating the local datums to mean high water springs (MHWS).

Normal-orthometric heights can be related to the NZVD2009 or NZGD2000 by using a the NZGeoid2009 geoid model and the appropriate datum offset. The conversion of heights between the different datums is described on the height conversion pages.

The accuracy of NZVD2009 heights is described by a series of Orders. Note that the Orders used for normal-orthometric heights are different to that used for ellipsoidal heights.