Module 7 - Charts & Publications
Interpret Symbols, Soundings, Tints, and Drying Heights
What you will learn
Use the correct symbol reference and chart legend to interpret paper-chart symbols, depth soundings, drying heights and colour tints without relying on a memorised colour shortcut.
NP5011 and INT 1 explain paper-chart symbols; the chart title and notes supply the units and datums that give each number meaning. Electronic portrayal needs its own maintained symbol key.
ADMIRALTY NP5011 gives the symbols and abbreviations used on ADMIRALTY and International Chart Series paper charts; IHO INT 1 is the international chart-user key. These references cover hydrography, topography, hazards, aids and services. Symbols on an ENC or leisure plotter may be portrayed differently, so use the maintained symbol key or Chart 1 for that system rather than assuming the paper symbol is identical.
Colour supports interpretation but is not a complete rule. On familiar multicolour paper charts, tints help distinguish depth bands and drying areas, but the relevant contour values, legend and notes control the meaning. Electronic day, dusk and night palettes can alter appearance. Never decide that water is safe simply because it is white, blue or untinted.
A sounding is a charted depth referred to the chart's stated vertical datum and units. On a modern metric UK chart, 3.2 commonly means 3.2 metres below Chart Datum. A drying height is an elevation above Chart Datum, commonly shown as an underlined figure on paper charts. Actual or predicted water depth still depends on tide, weather, uncertainty and the vessel's clearance policy.
Worked example
A 3.2 m sounding and 1.5 m predicted height of tide are used for an initial depth estimate.
- 1Confirm both values are metres referred to the intended Chart Datum and time.
- 2Add 3.2 + 1.5 = 4.7 m predicted depth.
- 3Compare 4.7 m with draught, squat or heel where relevant, survey uncertainty and the skipper's UKC policy.
Sense check: Rising tide increases water above a sounding; the arithmetic should not make the predicted depth smaller.
| Chart item | Meaning to establish | Decision check |
|---|---|---|
| Sounding | Depth below the stated Chart Datum in the stated units | Add the applicable height of tide, then allow for uncertainty and UKC |
| Drying height | Feature height above Chart Datum | Subtract it from height of tide to estimate water over the feature |
| Depth contour | Boundary of a charted depth band | Relate it to draught, tide, survey quality and route margin |
| Colour tint | Visual support for a band or feature | Confirm with legend, contours, notes and display palette |
| Symbol or abbreviation | Meaning defined by the relevant chart reference | Use NP5011/INT 1 for paper or the system's maintained ENC key |
Turn chart marks into a depth decision
The route crosses a charted sounding of 2.4 m near a drying feature marked 1.2 m. The predicted height of tide is 1.8 m.
1. Confirm reference
Check that the chart uses metres and identify its Chart Datum and relevant cautions.
2. Sounding
Predicted depth at the 2.4 m sounding is 2.4 + 1.8 = 4.2 m before other effects.
3. Drying feature
Predicted water over the 1.2 m drying height is 1.8 − 1.2 = 0.6 m.
4. Make the decision
Apply uncertainty, actual observations, draught and the chosen under-keel-clearance margin.
Sense check: The same height of tide is added to a sounding below Chart Datum but reduced by a drying height above Chart Datum.
Common mistake or limitation
- Treating a tint as a universal safe-water or danger code without reading the chart's contours and legend.
- Assuming NP5011 is the complete key for every electronic display; ENC and plotter portrayal needs its own current reference.
- Reading a drying height as water depth or forgetting that a predicted depth is not guaranteed clearance.
Recap
- Use the symbol reference that belongs to the chart or display.
- Numbers need units and a vertical datum before they have navigational meaning.
- Tint supports contours and symbols; it never proves safe water on its own.
Optional quick check
Section 2 of 5
A metric chart shows a 2.4 m sounding and the predicted height of tide is 1.8 m. What is the initial predicted depth?
Sources and factual review
Continue studying Charts & Publications
This public lesson is part of Revision Module 7. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.