Sea level heights

Sea level has traditionally provided a convenient reference for location vertical datums (LVD). Geoid-based datums like New Zealand Vertical Datum 2016 (NZVD2016) are determined independently of sea level, and are now the preferred reference.

A vertical datum is a reference system (or set of rules) that defines how heights are represented. The most convenient way to establish a vertical datum has traditionally been to reference it to a sea surface, commonly known as sea level.

However, accurately determining sea level can be difficult. Sea level is affected by topography, currents, weather and salinity. On top of this, a complete tidal cycle takes approximately 18.6 years, but most tide gauges have recorded data for far shorter periods. This means that data from different reference points or time spans cannot easily be combined.

Long-term changes in sea level also mean that the mean sea level reference point and the current mean sea level gradually misalign. For example, in 2017 mean sea level in Wellington Harbour was 0.22 metres above the Wellington Vertical Datum 1953.

Various sea level surfaces exist, with the most common being mean sea level (the average height of the ocean's surface). Different applications will specify specific surfaces:

  • topography – mean sea level
  • hydrography – lowest astronomic tide and chart datum
  • cadastral – mean high water springs
  • geodesy – mean sea level.

Definitions of each of these primary tidal surfaces can be found in our Tides glossary.

Tides glossary

Modern vertical datums, such as New Zealand Vertical Datum 2016 (NZVD2016), are determined using global gravity models and enhanced using localised gravity observations. These datums can provide national consistency and a more reliable foundation for most applications such as surveying, mapping and disaster resilience planning.

New Zealand Vertical Datum 2016

Following the introduction of NZVD2016 and its vertical datum relationship grids, we can transform data captured in terms of land-based vertical datums to a common reference.

Vertical datum relationship grids

We have not yet developed a relationship grid for transforming data associated with sea surfaces, but the Joining Land and Sea (JLAS) project aims to consolidate data along Aotearoa New Zealand’s coastline. JLAS will support greater resilience to natural hazards by helping to model changes in sea levels, flooding and tsunami inundation.

Joining Land and Sea (JLAS) project

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