Above Mean Lower Low Water (MLLW) signifies a water level that is higher than the average height of the daily lowest tide observed over a 19-year period. This "average of the daily lowest tide" is what NOAA defines as Mean Lower Low Water (MLLW). Understanding this concept is important when dealing with tide predictions and nautical charts.
Understanding Mean Lower Low Water (MLLW)
As mentioned in the provided reference, MLLW is the baseline used by NOAA to measure water depths and tide heights. It's calculated by:
- Identifying the lowest tide each day: Over a span of 19 years, the lowest water level for each day is recorded.
- Calculating the average: The average of all these daily lowest tides over the 19-year period provides the MLLW value.
Here is a basic table illustrating this idea:
| Term | Definition | Example |
|---|---|---|
| Mean Lower Low Water (MLLW) | Average height of the daily lowest tide over a 19-year period. | 0 feet on a nautical chart |
| Above MLLW | A water level higher than the MLLW datum. | 3 feet means the water level is 3 feet above the average daily lowest low tide. |
Practical Implications
- Navigation: When a chart states a depth at 5 feet, this means that the depth at MLLW is 5 feet. Therefore, when the tide is above MLLW, there will be more than 5 feet of water at that location.
- Tide Predictions: When tide predictions are given, the height is often given in reference to MLLW. For example, a high tide of 4 feet above MLLW means the water is 4 feet higher than the average lowest low tide.
Key Takeaways
- "Above MLLW" indicates that the water level is higher than the average of the lowest low tides.
- MLLW serves as a vital baseline for nautical charts and tide prediction.
- Always check the reference point (MLLW) when interpreting tide information.
