Decide
In tidal waters, what level is chart datum usually set to?
The lesson in plain words
- Chart datum
- Chart datum (CD) is the level from which charted depths are measured.
- In tidal waters it is usually LAT, the lowest astronomical tide: about the lowest level the tide reaches in normal weather.
- So there is nearly always more water than charted, but high air pressure or offshore winds can push the sea lower.
- The title notes of each chart say which datum it uses.
- Depths
- A sounding is a charted depth below chart datum.
- Check the chart's stated units: metres, fathoms or feet.
- On a metric chart, a large 4 followed by a smaller 2 means 4.2 m.
- Depth contours join equal depths.
- Actual depth = charted depth + height of tide.
- The Tides and heights unit shows how to find the height of tide.
- Drying heights
- Areas that cover and uncover with the tide are usually coloured green.
- Their figures are drying heights, shown underlined: an underlined 2 with a small 1 means the seabed there stands 2.1 m above chart datum.
- With a height of tide of 3.0 m, that spot has only 0.9 m of water over it.
- Heights and clearances
- Land heights and light elevations (heights) use a stated high-water level, often MHWS (mean high water springs, average high water at spring tides).
- Modern Admiralty charts normally reference vertical clearances (room overhead) to HAT (highest astronomical tide, highest predicted astronomical tide level); other charts may use another level. Check every chart's notes.
- Weather can raise water above HAT and reduce clearance.
- For overhead power cables, use the stated safe clearance, allowing for electrical discharge, rather than physical height alone.
- Keep a margin
- Charted depths come from surveys that may be old or patchy, and sand moves.
- Keep a safety margin under the keel, larger over rock, in waves or where the survey is old.
Key data
- Chart datum (tidal waters)
- Usually LAT, lowest astronomical tide
- Actual depth
- Charted depth + height of tide
- Water over a drying height
- Height of tide − drying height
- Light elevations
- Above a high-water level, usually MHWS
- Bridge and cable clearances
- Normally HAT on modern Admiralty charts; check every chart’s notes
Key points
- Depths are below chart datum, usually LAT in tidal waters.
- Actual depth = charted depth + height of tide.
- Underlined figures are drying heights above chart datum: water over = height of tide − drying height.
- Heights often use MHWS; modern Admiralty clearances normally use HAT. Check every chart’s notes.
- Read the chart's title notes for its datums and units.
Check what you have read
How is a drying height shown on a chart?
True or false?
Decide each statement before you see the answer.
A bridge clearance is measured from chart datum.
Actual depth = charted depth + height of tide.
There can never be less water than charted.
Heights often use MHWS; modern Admiralty clearances normally use HAT. Check every chart’s notes.
Chart datum is the average sea level.
Underlined figures are drying heights above chart datum: water over = height of tide − drying height.
Practice case & exam keys
Situation
You plan to anchor for lunch where the chart shows 1.5 m. Your keel draws 1.8 m. The height of tide is 2.4 m at noon and falls to 0.6 m by 18:00. Is it a good spot for the afternoon?
Show the answer
At noon the depth is 1.5 + 2.4 = 3.9 m, leaving 2.1 m under the keel. By 18:00 it is 1.5 + 0.6 = 2.1 m, only 0.3 m under the keel, and less in any swell. That margin is too small: anchor in deeper water or leave well before the tide falls.
Exam keys
- Chart datum, usually LAT in tidal waters; depths are measured below it.
- Actual depth = charted depth + height of tide.
- Drying heights are underlined and measured above chart datum.
- Light elevations often use MHWS; modern Admiralty clearances normally use HAT. Check every chart's notes.