Measuring Sea Level: Deceptively Simple, Incredibly Precise

Normally a job for governments, the Hakai Institute has installed a new permanent tide station at the Calvert Island Ecological Observatory.

You walk down to the edge of the water. A light breeze hits your skin and the ocean tickles your toes with each passing wave. Gulls caw repeatedly as they fly overhead. The smell of salt and drying seaweed fills the air. To ensure your favorite tide pools aren’t underwater, you checked the tide table before you left. But who made the tide table? How did they do it?

The Canadian Hydrographic Service (CHS) has been measuring tides and currents in BC waters since 1893 to facilitate safe navigation for commerce and recreation. Over that history, it has become apparent that sea level is rising at an increasing rate and so monitoring stations globally must now measure to within one millimeter according to standards set by the United Nations. Meeting this standard is typically left to government agencies, like CHS, which makes the new permanent tide station at Hakai’s Calvert Island Ecological Observatory the first privately operated tidal station in British Columbia, and likely in all of Canada.

“It is a great plus that Hakai has put the tide gauge in that location,” says CHS physical scientist Neil Dangerfield. He adds that the site in Pruth Bay on Calvert Island will “absolutely fill a gap” between existing CHS tide stations in Port Hardy and Bella Bella.

Some locations between these distant gauges do have tidal predictions, but predictions are based on past, temporary measurements and no instrumentation is currently in place. Therefore, weather events, such as storm surges, may cause deviations from predicted tidal heights by up to a meter. Such variations are hazardous to navigation in the wrong circumstances, and are justification for the nearly 40 tidal stations on the BC coast, mostly located near ports.

The Hakai Institute is interested in aiding navigation safety, however, a primary motivation for the tidal station in Pruth Bay is to improve the data quality for mapping the seafloor and marine ecosystems around Calvert Island.

“Only about one third of BC’s coastal waters have been mapped with modern multibeam echosounders,” says Jacques Gagne, a lead hydrographer with CHS. The new tidal station will ensure that Hakai’s mapping efforts will meet CHS standards and our work will fill some of these gaps.

Tidal observations rely on specialized gauges that measure the water level by one of three methods: the distance to the water below a fixed point, the level of the water itself, or the pressure of the water above the seafloor. Earlier this spring, Hakai staff bolted two new gauges, a radar and a pressure water level gauge, to the Calvert Island pier. The radar system is designed to bounce a signal off the water surface every second and “listen” for the return, like a bat or a whale using echolocation. The pressure gauge, strapped to the base of the pier piling by Hakai divers, is functionally similar to a bathroom scale. The two independent and redundant systems can attain the millimeter precision and eliminate potential sources of measurement error as required by CHS and the United Nations.

One final piece is needed to get the tidal height—the position of the gauge relative to an established geodetic mark, a surveyed permanent marker on nearby land. In June, the hydrographic survey vessel CCGS Otter Bay pulled up to the Calvert dock. Its three person CHS team spent two days at the observatory surveying the precise (+/- 1 mm) horizontal and vertical geographic location of the new gauge. CHS is currently working to match up data from the new sensors with established tide stations, the final step of the calibration process.

When calibration is completed, data produced from the Pruth Bay gauges will be publicly available through the CHS website to be used by anyone. From there, data will be integrated into global oceanographic models, used to determine trends in local sea level rise, and will improve local navigation for mariners from pleasure craft to cruise ships who can use it in their planning.

The public can take tidal predictions for granted, but it takes effort and expertise to provide the information that facilitates safe commerce, navigation, and recreation. Thankfully, there are those able and willing to do the job that allows the rest of us to enjoy the sea air as the waters rise and fall.