What is mean low water based upon?

Mean low water is defined as the average of all the low water readings recorded over a specific timeframe, which is typically 19 years. This measurement serves as an important reference point in tidal charts and nautical navigation, allowing mariners to understand the typical low water levels they can expect in a given area. By using a period of 19 years, the calculation accounts for variations due to seasonal changes, weather patterns, and other environmental factors, ensuring a reliable average that reflects typical tidal behavior over an extended period.

Although other choices mention different timeframes or methods for determining low water levels, they do not align with the standardized practices used in tidal measurement. Historical low tides and the lowest recorded tide each year do not provide the comprehensive average required to establish mean low water.