What is used as chart datum on almost all charts?

Mean Lower Low Water is utilized as chart datum on nearly all nautical charts because it provides a consistent reference point for mariners to gauge water depth and navigate safely. This datum represents the average of the lower low water heights observed over a specific period, ensuring that navigators account for the lowest expected water levels in tidal areas. By using this reference, charts can display the least depth for safe navigation, minimizing the risk of grounding during low tide conditions.

In contrast, options like Mean Sea Level and Mean High Water do not account for the lowest possible water levels, which could mislead mariners about safe depths in waterway navigation. Mean Tidal Range, while important for understanding tidal variations, does not provide a fixed reference for charting purposes. Thus, using Mean Lower Low Water ensures that navigational charts offer accurate and safe information necessary for maritime operations.