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
Last reviewed: 2026-07-08Richard Moore
Depths, drying heights, clearances
Chart Datum and sea level
Drag the tide level to see why charted depth, drying height, tide height, draught, and bridge clearance use different references.
Tide height
2.7 m
Water depth
4.5 m
Under keel
3.0 m