Chart datum and tide predictions

[Fisherman Rob]

Correct me if I am wrong, but I understand Canadian charts and tide tables both provide heights based on the same chart datum? So at a specific tidal station, if depth on a chart is 2.1m and the low tide is a 1.6m tide according to the tide table, then the actual height of the water column at that location at low tide will be 2.1+1.6 = 3.7m.

This calculation would only be accurate at the tidal station itself, and for any other location, you would have to guesstimate given the distance from the tidal station.

 

[vanisland]

Yes your calculation is correct, the charts show the lowest possible depths.

 

[Kisinana]

This is from the Canadian Government Site

For navigational safety, depths on a chart are shown from a low-water surface or a low-water datum called chart datum. Chart datum is selected so that the water level will seldom fall below it and only rarely will there be less depth available than what is portrayed on the chart. The following three criteria place somewhat more restriction on its choice: chart datum should be:

1.so low that the water level will but seldom fall below it,
2.not so low as to cause the charted depths to be unrealistically shallow, and
3.it should vary only gradually from area to area and from chart to adjoining chart, to avoid significant discontinuities.
On most Canadian coastal charts the surface of lower low water, large tide, or LLWLT , has been adopted as chart datum, but the term " lowest normal tide, " or " LNT, " has been retained on the charts since it encompasses a variety of other choices for chart datum on some older charts.