Augmenting the utility of NSW longest tide gauge records with continuous GNSS technology

The tide gauge records available from Fort Denison, Sydney Harbour (since 1886) and the Pilot Station, Newcastle (from 1925) in NSW are two of the longest continuous tide gauge records in the southern hemisphere. By virtue, these records are of considerable international importance in relation to sea level rise research.

Tide gauge data provides a measure of the water level “relative” to a fixed, land-based reference mark. What is often not known, however, is whether or not the land upon which the tide gauge is positioned is actually moving. There are in fact many processes that can contribute to the land moving including solid earth tides, plate tectonics, glacial isostatic adjustment and localised factors that contribute predominantly to subsidence (e.g. aquifer extraction, reclamation and development loadings). In order to determine a rigorous estimation of sea level change, these processes need to be taken into account, i.e. is the sea level rising or the land falling? Although other researchers had noted discrepancies between the Fort Denison and Newcastle tide gauge records, Watson (2011) identified that the Pilot Station gauge record was likely contaminated by subsidence due to a range of separate coal mine workings at depths of 20-70 m in the vicinity of the gauge site.

During the past 12 months, various NSW Government agencies have worked collaboratively to install a Global Navigation Satellite System (GNSS) Continuously Operating Reference Station (CORS) at both gauge sites in order to accurately and continuously measure landform movements. With accurate time series knowledge of the landform movements, “relative” tide gauge measurements can be readily adjusted to measure the absolute changes in ocean water levels over time. The GNSS CORS installed at Fort Denison and Pilot Station, Newcastle, have been integrated into the state government’s CORSnet-NSW network managed by Land and Property Information to deliver high-accuracy positioning data for NSW.

This paper outlines the scientific value associated with augmenting these long tide gauge records with GNSS and provides an insight into the difficulties encountered in considering the installation of these units in sensitive, heritage-listed, harsh marine environments.