Height Interpolation

What it does

• Tide height between high and low water follows a curve, not a straight line.
• Six-hour interpolation over-reads depth near mid-tide by up to 20 per cent.
• The tool returns the corrected height and the time of the same height on the next cycle.

How to use it

1. Always interpolate between two known extremes; never extrapolate past the last entry.
2. Write the height to two decimals before rounding for a draft decision.
3. If the two heights differ by less than 0.4 m, treat the day as level water.
4. Cross-check the result against the nearest port observation before a shallow entry.

Safety notes

• A computed height is not a depth sounding; allow for chart datum error and swell.
• Add the full swell forecast to your required depth, never the average.