Chart Datum Shift

What it does

• Three datums appear on one small-craft chart and they are not interchangeable.
• A one-line shift prevents the common habit of adding the whole tidal range twice.
• The tool keeps the sign explicit, which is where most datum errors start.

How to use it

1. Write the datum name next to every height you copy into a plan.
2. A gauge offset is a local fact, not a chart fact; record who supplied it.
3. When two sources disagree, believe the one with a stated datum.
4. Keep the shift value on the plan so a later reader can reproduce it.

Safety notes

• Do not mix a gauge reading with a chart height without a stated offset.
• If the datum is unknown, treat the number as unusable for a depth decision.