Levelling connections between GNSS sites and tide gauges

Geoscience Australia is the custodian of levelling connections between Global Navigation Satellite System (GNSS) sites and tide gauges. The levelling connections are undertaken by state and territory land and survey departments and sent to Geoscience Australia to make them publically available.

Why are levelling connections important?

In most cases, three types of measurement are necessary to assess whether sea level is changing at a given point: tide gauge data, GNSS time series and levelling observations.

Tide gauges provide a measure of relative sea level variation, that is, how sea level changes relative to a tide gauge attached to a rock wall or wharf. A tide gauge cannot however differentiate between changes in the sea level and movement of the tide gauge. For example, if the tide gauge is fixed to land that is subsiding, this causes relative sea level rise. The sea level itself however may not have changed.

GNSS can be used to measure the vertical crustal motion of the Earth with respect to the centre of the Earth. This makes it a useful technique to distinguish relative sea level rise from absolute sea level rise. For example, in the case of the tide gauge subsiding, if the land subsidence has been observed by GNSS, this trend can be removed from the relative sea level variation and an estimate of absolute sea level change can be derived.

In situations where a GNSS station is not attached directly to the tide gauge, in order to distinguish the sea level variations from vertical crustal movement, it is necessary to regularly measure the height difference between the GNSS station and the tide gauge. Any variation in the height component of the GNSS time series or change in height between the GNSS station and tide gauge will have an impact on the measure of absolute sea level change at a tide gauge.

Data

Below is a list of the levelling connections Geoscience Australia holds on behalf of the Australia's state and territories. For further information, please contact geodesy@ga.gov.au

For GNSS data relating to these sites, please visit the Geoscience Australia GNSS Data and Site Logs webpage