What is meant by "chart datum" in navigation?

"Chart datum" refers specifically to a reference point or surface from which depths are measured on navigational charts. It serves as a baseline that helps navigators determine the actual water depth in a given area, ensuring safe navigation by indicating how deep the water is relative to this standard level.

In nautical contexts, chart datum often corresponds to the lowest astronomical tide (LAT), meaning depths shown on charts are often referenced to this lowest level. This is crucial for safe operations, especially in shallow waters, where understanding the exact depth is essential for avoiding groundings and navigating safely.

The other options do not correctly define chart datum. A level used to measure weather changes pertains to meteorological measurements, a standard for chart quality concerns the accuracy and reliability of the chart itself, and a type of navigation calculation would refer to methods used to derive a position, which does not relate to the concept of depth measurement in navigation.