14 Jul 2016

Exactly where is sea level? Gravity can tell us

From Our Changing World, 9:06 pm on 14 July 2016
Lake Pukaki view

To accurately measure the height of mountains, such as these near Lake Pukaki, you need to know precisely where sea level is - and the new vertical datum measures that. Photo: CC BY-SA 2.0 Joaquin Corbalan

Rachelle Winefield amd Graeme Blick

Rachelle Winefield amd Graeme Blick, from LINZ, next to a tidal gauge on the Wellington waterfront that measures local tide heights but is not accurate enough to measure elevation from. Photo: RNZ / Alison Ballance

The summit of Aoraki / Mount Cook is 3,724 metres above sea level, which begs the question: where exactly is sea level?

Land Information New Zealand (LINZ) has just released a new vertical datum which defines exactly where sea level is, right across New Zealand and its coastal seas.

LINZ says that understanding the height of our land is important for understanding how water will flow during a flood or storm. People commonly talk about metres above sea level as a way of describing heights, but sea level can vary at different points around the country. To give scientists and researchers consistency, LINZ has developed a new vertical datum, a reference system based on gravity.

This has been four years in the making and has involved flying the length and breadth of the country with a specially equipped plane for gathering gravity measurements, as gravity can vary from place to place.

Survey mark

Surveyors manually calculate elevations with reference to survey marks, such as this one, that are scattered across the country. Photo: RNZ / Alison Ballance

The new vertical datum, based on these gravity measurements, is accurate to 3cm. This enables, for example, scientists to reliably measure changes in sea level, engineers to consistently locate underground services and surveyors to efficiently define three-dimensional property boundaries.

Graeme Blick and Rachelle Winefield from LINZ have been responsible for collecting the gravity data and creating the vertical datum.

Geologists say they use the gravity data to investigate rocks and the structure of the earth beneath the surface, where they can't see..

In this earlier Our Changing World story, Graeme Blick explains geodesy, the science of knowing where you are.

The Deepwave project visited New Zealand to measure gravity waves in the atmosphere.

Gary Wilson from the University of Otago was recorded in Antarctica, measuring gravity with a very sensitive meter.


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