When navigating a ship the number one objective is to not sink the ship. To achieve this, it of course helps if the ship does not run into anything above or below the water surface but it is not always possible to see submerged objects coming up in time to navigate around them and so the navigator of a ship will refer to a chart in order to avoid obstacles. These charts contain 2 pieces of information, the horizontal position of features, and their associated depth in relationship to what is called the chart datum.
Chart datum is defined in Canada as the Lower Low Water, Large Tide (LLWLT) which is to say that the depth is relative to the yearly average of the predicted lowest water level for a 19 year lunar cycle based on observations from tide gauges. The water depths are then determined through bathymetric surveys in relation to these tide gauges. With the advent of GNSS the ocean mapping community saw the advantage to perform their surveys in relationship to the ellipsoid and then linking these heights to the tide gauges, this in turn lead the Canadian Hydrographic Service (CHS) to create a seperation model (SEP) between the chart datum and the ellipsoid by measuring the separation with GNSS on the tide gauges.
Of course the ocean system is dynamic, meaning that the water levels measured at a tide gauge are only relevant to those waters nearby (less than 10 km) as water levels are not a linear function of distance. This is due to different factors that change over time such as the chemical composition of the water, the currents, etc.. This creates a problem as it would be unrealistic to have a complete network of permanent tide gauges covering the entire Canadian coastline (stretching over 200,000 km it is the longest in the world) and re-survey these waters, tieing them into the tide gauges.
And so in 2010 the CHS and the Canadian Geodetic Survey (CGS) to examine the possibility of creating a model that would represent the continuously changing water levels which would be called the Continuous Vertical Datum for Canadian Waters. Such a model will aid in defining; coastlines, inter-tidal zones, maritime boundaries, marine cadastres, claims to sovereignty. As well as serving as a baseline for sea level rise & related climate change adaptation strategies, and improve coastal infrastructure maintenance & development.
To get an idea of just how complex such a model is, the input data for the model includes; the Canadian Gravimetric Geoid of 2013 (CGG2013), GNSS observations, Satellite Altimetry, Water Level Observations, Ocean Models, and Coastline Surveys.
As you are starting to gather by now, this is an extremely complex project that encompasses the most challenging of problems over multiple fields and disciplines. Aka a perfect topic for the Geodesy Corner and I look forward to exploring this further in the weeks to come.
Catherine Robin, Shannon Nudds, Phillip MacAulay, Andre Godin, Bodo De Lange Boom & Jason Bartlett (2016) Hydrographic Vertical Seperation Surfaces (HyVSEPs) for the Tidal Waters of Canada. Marine Geodesy, 39:2, 195-222, DIO:10.1080/01490419.2016.1160011