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In a previous post, we looked at some of the basic principles involved with understanding tides, including what causes their rise and fall, the resulting tidal streams and the tidal range that varies according to coastal topography. For those who are learning or revising how to sail in tidal waters, getting to grips with all the terminology and definitions can seem a little daunting at times; even experienced sailors need to do a bit of revision from time to time. In this post, therefore, we look at some of the many terms and definitions associated with the language of tides. We will look into greater detail in future posts.

Chart datum

Chart datum is the level from which depth soundings and drying heights are measured, below the minimum height of low water springs. It represents the lowest level to which the tide will ever fall, taking astronomical factors into consideration rather than meteorological ones and is therefore referred to as the Lowest Astronomical Tide (LAT). Note it is possible that exceptionally strong winds can raise the height of tide and exceptionally high pressure can depress it, but these are unpredictable factors.

Mean heights above chart datum

The mean heights above chart datum of high and low water at spring and neap tides are shown in nautical almanacs, pilot books and on charts.  These are abbreviated as follows:

  • MHWS – Mean High Water Springs
  • MHWN – Mean High Water Neaps
  • MLWN – Mean Low Water Neaps
  • MLWS – Mean Low Water Springs

Charted depth

The charted depth refers to the depth, or amount, of water that will remain beneath a given position on a nautical chart if the tide level falls to zero. Charted depths are measured in feet or metres and shown on a chart by contour lines or soundings.

Charted height

The charted height is the height of a given feature, such as a bridge, above Mean High Water Springs (MHWS). The highest possible tide taking astronomical factors into account is referred to as Highest Astronomical Tide (HAT). This helps navigators calculate whether their vessel can pass beneath a bridge at high water. Should this not be possible they may be able to pass beneath the bridge at low water, which navigators can also calculate.

Height of tide

The height of tide refers to the vertical distance between the actual height of water at a specific point in time and the level of chart datum. The height of tide can be calculated at any given time between high and low water by referring to tide tables and nautical almanacs, using a tidal curve for the calculation. Explaining how this is done will be covered in a future post.

Drying height

The drying height refers to a charted feature or area of the sea bed that is covered at MHWS and uncovered when the tidal height reaches a specific height above chart datum, for example 2.6m

Tidal range

The vertical distance between the levels of low water and and the next high water, or vice versa.

Rule of twelfths

The rule of twelfths is a simplified, quick method for navigators to estimate the height of tide at a particular location. This gives a useful approximation and assumes that the calculation is being done in an area where the rise and fall of the tide is regular, which is not always the case. The rule of twelfths assumes that the rise starts slowly, then speeds up and then slows down over a six hour period. The rule of twelfths will be explained fully in a future post.

tidal terms and definitions



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