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Wednesday, October 20, 2010

US and Canadian Tides

Here in the Salish Sea region we are frequently (in my case, daily) traveling between the waters of Washington and British Columbia. It is critical that we understand the differences between the conventions used on US charts and tide tables and Canadian charts and tide tables for this region, because in many cases they overlap.

In addition to the fact that US and Canadian charting agencies use different sounding units (metric in Canada, imperial in the US, alas), US and Canadian charts also use a different reference point for their soundings.

US charts and tide tables are based on Mean Lower Low Water (MLLW), that is, the average of all of the daily lower low water heights over a 19 year metonic (earth/sun/moon) cycle. By definition then, 50% of the lower low waters over that same 19 year period will be "minus tides", meaning that they are below the chart datum.

Canadian charts and tide tables, on the other hand, use Lower Low Water Large Tide (LLWLT), also called Lowest Normal Tide (LNT). LLWLT is the average of the 19 lowest low waters for the same 19 year metonic cycle, or rather the average of all of the lowest tides for each year over that same period. So it is still possible to have a tabular "minus tide" in Canadian waters, but for any given tide station you should see no more than 10 of these total over a 19 year cycle, as opposed to some 3500 minus tides for the same location on US charts for the same time span. 

In both cases, the Tide Tables inherently compensate for this datum difference, so if you were to take the combined charted depth and tide at a given moment for a given station from both US and Canadian sources (and convert from metric to imperial), the total depth of water would be presumably exactly the same. 

However, if we mix US tide tables with Canadian charts, we will find that we have significantly more water at a given location than what we have computed. This is mostly only a problem for bathymetric navigation. 

The much worse scenario is that if we mix Canadian tide tables with US charts, we will actually have significantly less water than what we have computed. The first scenario is a navigational embarrassment, the second may well result in a grounding.  

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