A nautical chart shows depths, drying heights and clearances, and tide tables give heights of tide. They only make sense together because they all refer to the same reference level: chart datum. This guide defines each quantity, says which direction it is measured in, and shows how to combine them. If you learn one thing, learn the direction of each arrow in the diagram.
Cross-section showing chart datum, charted depth, height of tide, drying height, draught and under-keel clearance.
Chart datum
Chart datum is the level from which everything tidal is measured. Different charting authorities choose different levels. UK Admiralty charts, and most charts following IHO practice, use Lowest Astronomical Tide (LAT): the lowest tide level that can be predicted under average meteorological conditions. U.S. NOAA charts use Mean Lower Low Water (MLLW): the average of the lower of the two daily low waters over a 19-year period. The choice matters for one practical reason. Below LAT there is, by definition, almost never less water than charted; below MLLW there frequently is, because MLLW is an average and many low waters fall below it. Tide tables for each country give heights relative to that country's chart datum, so the sums work the same way — but the safety margin implied by “zero” differs.
The datum can also differ from chart to chart within one area, especially on older or large-scale harbour charts. The chart's title block states the datum and, where relevant, the tidal levels at named places. Read it.
The five quantities
| Quantity | Measured | Where you find it | Chart symbol |
|---|---|---|---|
| Charted depth (sounding) | Downward from chart datum to the seabed | Chart | Plain figure, e.g. 5₂ = 5.2 m |
| Height of tide | Upward from chart datum to the sea surface at a given time | Tide tables / tidal curve | — |
| Drying height | Upward from chart datum to the top of a feature that uncovers | Chart | Underlined figure, e.g. 1₅ = dries 1.5 m |
| Draught | Downward from the waterline to the deepest part of your keel | Your boat | — |
| Clearance height (bridges, cables) | Upward from a high-water datum — usually HAT on modern Admiralty charts (older ones use MHWS); MHW on NOAA charts | Chart | Figure in brackets or with a bar over it |
Clearance heights are the trap. They are measured from a high-water level, not chart datum, so a bridge charted at 12 m has at least 12 m of clearance at that high-water level and more at any lower tide. To find the clearance at a given time you need the height of the datum used above chart datum (given on the chart or in the tide tables) and the height of tide. Check the chart legend for which datum applies before you do the sum.
The three sums
- Depth of water over a sounding: actual depth = charted depth + height of tide.
- Depth of water over a drying feature: actual depth = height of tide − drying height. If the answer is negative, it is dry.
- Under-keel clearance: actual depth − draught. Decide your minimum in advance and stick to it — half a metre is a common minimum in calm water; more in swell.
Worked example
You want to anchor in a bay charted at 1.8 m near a rock that dries 0.9 m. Your draught is 1.6 m. Tonight's low water is 0.7 m above chart datum. At low water the anchorage will have 1.8 + 0.7 = 2.5 m of water, giving 0.9 m under the keel: acceptable in calm conditions. The rock will have 0.7 − 0.9 = −0.2 m: it will be showing by 0.2 m, which is helpful, because you can see it. Your swinging circle must keep the boat clear of it at any state of tide and wind.
The echo sounder is not the chart
An echo sounder measures the actual depth now, from wherever its transducer is (or from the keel or the waterline, depending on how the offset is set). It already includes the height of tide. Adding a height of tide to an echo-sounder reading double-counts the tide. Use the sounder to check the chart — sounder reading minus height of tide should match the charted depth — and to watch the water under you, not as an input to the datum sums.
The common mistake
Mixing quantities: treating a drying height as a depth, treating a clearance height as if it were measured from chart datum, or adding a height of tide to an echo-sounder reading. The cure is mechanical. For every number, ask “measured from what, in which direction?” before you use it. If two numbers are not measured from the same level in a way that lets them add or subtract, convert one first.
Check yourselfA sill at a marina entrance dries 2.0 m. Your draught is 1.5 m and you want 0.5 m clearance. What height of tide do you need, and how would you express the water over the sill when the height of tide is 3.2 m?Show answer
You need 1.5 + 0.5 = 2.0 m of water over the sill, so a height of tide of 2.0 (drying height) + 2.0 = 4.0 m. When the height of tide is 3.2 m, the depth over the sill is 3.2 − 2.0 = 1.2 m — less than your draught, so you would go aground.
Key takeaways
- Chart datum is the reference for depths (down) and heights of tide and drying heights (up). UK: LAT. U.S.: MLLW.
- Actual depth = charted depth + height of tide. Water over a drying feature = height of tide − drying height.
- Clearance heights use a high-water datum (HAT or MHWS in the UK, MHW in the U.S.), not chart datum — check the chart legend.
- The echo sounder reads actual depth now. Never add the tide to it.
