The correction method
The same tide stage. A different clock time.
A tide atlas uses stages relative to Portsmouth high water. The actual intervals between high and low water vary, so a fixed clock offset is only a reference.
Illustrative arithmetic · not a dated tide record
- Standard time
- 12:00
- Correction
- −20 min
- Corrected time
- 11:40
Read a correction in three steps.
- 1. Identify the reference. HW means high water; LW means low water. The sign in a stage label indicates before or after that reference event.
- 2. Add the correction. Add the signed number of minutes to the standard time. A minus sign shifts it earlier; a plus sign shifts it later.
- 3. Check the date. A corrected time can cross midnight. Keep the date note with the time, and cross-check against official local information.
Why does the interval change?
The dominant lunar tide repeats roughly every 12 hours 25 minutes. The solar tide repeats every 12 hours. Their interference changes the range and the shape of each cycle.
In shallow seas, friction and bathymetry introduce higher harmonics. They can prolong high water and compress another part of the cycle. A simple six-hour high-to-low interval cannot describe every day.
Explore the physicsWhat the result can tell you.
The selected-day table relates standard HW/LW stages to corrected times using the dated event sequence. It shows timing, not a new observation of the tidal stream.
High or low water is not necessarily slack water. A timing adjustment alone cannot validate an atlas stream vector, speed or direction. Wind, pressure and surge are outside this correction.
Read the full explanation