Where this fits
- Day Skipperrelevant
- Coastal Skipperrelevant
- Yachtmasterrelevant
Free tidal-height tool
Chart datum explained with a moving waterline
Understand chart datum, charted depth, drying height, height of tide, depth of water, bridge clearance, springs, and neaps. It is an independent revision aid, not official RYA training or assessment.
Direct answer
Chart Datum is the chart reference level from which charted depths and drying heights are measured. Depth of water is charted depth plus height of tide, while a drying height is land above Chart Datum that only covers when the tide rises above it.
- Best for
- Depth of water, drying heights, UKC, clearance
- Includes
- Springs and neaps slider
- Practice mode
- Datum cross-section, guided examples, quiz
- Cost
- Free, no login required
Depths, drying heights, clearances
Chart Datum: the zero line for depths
Drag the waterline. Watch which measurements are made from Chart Datum and which are made from a high-water clearance datum.
Goal: move the tide and ask which datum each measurement uses.
The bold CD line is the zero for depths and tide heights. Bridge clearance is measured from a separate charted datum.
Example tide range
Tide
2.7 m
Springs
Drag the tide bar (or focus it and use the arrow keys) to raise and lower the sea level. Springs have the larger range, neaps the smaller. CD sits close to LAT, so the tide rarely falls below the CD line - though weather can push it lower.
Height of tide
2.7 m
Charted depth
1.8 m
Depth of water
4.5 m
Draught
1.5 m
Under-keel clearance
3.0 m
Bridge clearance
8.3 m
Example chart: MHWS datum.
Link to this tool
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