Using GNSS CORS to augment long-term tide gauge observations in NSW

The tide gauge records available from Fort Denison, Sydney Harbour and the Pilot Station, Newcastle in New South Wales (NSW), Australia are two of the longest continuous records of this type in the southern hemisphere. By virtue, these records are of considerable national and international importance in relation to sea level rise and climate change research. Long-term tide gauge records provide a measure of the water level relative to a fixed, land-based reference mark. However, in order to precisely determine any apparent sea level change, it is important to know whether or not the land upon which the tide gauge is located is actually moving. Processes that can contribute to land movement include plate tectonics, glacial isostatic adjustment and localised factors that contribute predominantly to subsidence (e.g. groundwater extraction, land 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? Global Navigation Satellite System (GNSS) Continuously Operating Reference Stations (CORS) are the ideal candidate to provide accurate and continuous measurements of land movement at tide gauge locations. Following analysis of an accurate time series of this land movement, relative tide gauge observations can be readily adjusted to measure the absolute changes in ocean water levels over time. This paper outlines the high scientific value of augmenting tide gauge records with GNSS CORS and provides an insight into the considerable difficulties encountered in the installation of GNSS CORS in sensitive, heritage-listed, harsh marine environments. It focuses on the construction of two CORS installed at the Fort Denison and Pilot Station tide gauges, which have been integrated into the CORSnet-NSW network managed by Land and Property Information to deliver high-accuracy positioning infrastructure for NSW.