Height of Tide and Chart Datum

Height of Tide and Chart Datum
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Fig 5.10 – The depth of water at any point is the Charted Depth + the Height Of Tide. At the pinnacle of rock we can see that the Height Of Tide – Drying Height = the Depth of Water over the pinnacle of rock.

The Height of Tide is an important tidal level and refers to the level of the tide is at any point in time. The height of tide is measured from Chart Datum which can be considered the same as the lowest astronomical tide LAT.

Its not to difficult to see that if we have certain information we can calculate other details that we need for safe navigation. We will now go on to look at how to access this information and work out some of the common tidal height problems faced by sailors as they cruise around the coast.

The take home message is to realise that depths are measured from the lowest tide possible, and by doing so gives us a margin of safety, if we measure depths from CD/LAT, its not possible for the tide to get any lower, so gives us a worse case scenario.

Likewise when we measure clearances we measure from the highest tide we can get which is the rare Highest Astronomical Tide HAT. This gives us a margin of safety as the distance between the water level and the bridge at HAT cannot get any less.