In 2019, iXblue’s New Zealand office first deployed RBRvirtuoso³ D|tide16 loggers to measure the tides, in support of their high-resolution 3D sonar-generated maps of the seafloor. High–accuracy, long-term and low-drift tide measurements are required in iXblue’s work, to remove tides from survey data, to link geodetic land and ocean data and to model the tides in coastal regions.
iXblue specializes in nautical charting surveys for government hydrographic authorities and has been working with New Zealand’s authority, Toitū Te Whenua Land Information New Zealand (LINZ), since 2007.
As part of updating hydrographic data for LINZ, iXblue deployed RBR tide loggers to seven sites in 2019 during a survey of the Western Marlborough Sounds, a series of intricate inlets that, from space, resemble a fractal. In 2020, they deployed tide gauges to three sites for a survey of the Coromandel Peninsula, which extends 85 km northward into the Hauraki Gulf and shelters Auckland from the Pacific Ocean.
LINZ uses iXblue’s 3D seafloor maps and tide data to create detailed nautical charts. The tidal data, in particular, is critical for removing tides and waves from the surveys’ depth measurements. Tide records from near-shore locations are also used to create tide models, which increase the accuracy of the nautical charts.
“The tide gauges are typically deployed for at least 35 days, but sometimes up to four to six months,” says David Field, Operations Director based in New Zealand at iXblue. The 35-day minimum means they capture the full range of tidal changes in a lunar cycle with a buffer. They typically set the loggers to average over a two-minute high-frequency burst every five minutes, to remove waves from the tidal signal.
In 2019, iXblue began tidal data collection for LINZ’s three-year Joining Land and Sea project. iXblue is responsible for collecting sea level data and conducting tide gauge calibrations for 34 of New Zealand’s 89 collection sites. They collect this data using RBRvirtuoso³ D|tide16 tide gauges. The data will be used to constrain and calibrate a new tide model of New Zealand’s exclusive economic zone (EEZ), and interweave the country’s geodetic ocean and land datasets.
To collect tidal data, the iXblue team deploys RBR loggers in shallow water, mounted to wharves or to custom-made weighted craypots (lobster pots) that sit on the seafloor. The goal in both cases is to ensure the instruments remain motionless (and untouched by curious humans). The weights on the craypots ensure the frames do not shift, including preventing subsidence. Where there is a low likelihood that instruments will be disturbed, iXblue sets up cable connections to monitor the tidal data, in particular while surveys are ongoing. To add redundancy, they aim to deploy two gauges at each station. Field says, “If we can, we’ll have a download cable on one. The other one is a secondary, just in case the one with the download cable gets tampered with, we always have redundancy.”
iXblue was first introduced to RBR in 2018 through another contractor using RBR instruments. iXblue was looking to replace tide gauges which had batteries that could not be replaced or recharged. About the value of RBR instruments, Field says, “The sampling regime and the storage capacity, we know that the gauge is not going to run out of memory, it’s not going to run out of power. Because we can model our deployments mathematically and through the software, we can configure it correctly. So that’s probably one of the biggest reasons why we went with RBR.”
Sea level data has uses beyond supporting charts and tide models. Long term datasets contribute to sea level rise studies, modelling inundation scenarios during extreme weather events and help define regulations relating to construction in the coastal region.
Field says it’s also part of their role to share what iXblue sees on the seabed, to support science and underpin policymaking. For example, in some instances, iXblue’s 3D maps reveal trash or debris on the seabed. Unlike on land, where garbage is visible, the seabed is hidden from view. Field says more people understand there’s data and information available about the seafloor, and they want to see it and use it, which he sees as a positive from an ocean literacy perspective.
iXblue are now using RBRvirtuoso³ D|tide16 tide loggers to support their work for the Australian HydroScheme Industry Partnership Program, an Australian Department of Defence commercial acquisition program to undertake focused hydrographic survey activities in support of Australia’s national charting priorities.