At what level is Chart Datum set?

Chart Datum is a reference level used in hydrographic surveying and nautical charting to ensure consistency in depth measurement across different areas. It is typically set at the Lowest Astronomical Tide (LAT), which represents the lowest level that tides can reach under normal astronomical conditions. By establishing Chart Datum at this level, surveyors provide a reliable baseline that ensures all depth measurements account for variations in tide levels, and it guarantees that under no circumstances will the water depth go below the recorded depths on the chart.

Setting Chart Datum at LAT aids mariners in navigation by helping them to understand the minimum water depth. If chart depths were instead related to Mean Sea Level, for instance, they would not accurately represent the lowest depths that could be experienced.

Using LAT supports safety in navigation as it provides a standardized approach, ensuring that the mariners are aware of the absolute minimum depths they may encounter. Other options, like Mean Sea Level or Highest Astronomical Tide, do not offer this same level of assurance regarding vessel clearance and the risk of grounding. Therefore, the use of Lowest Astronomical Tide as Chart Datum is pivotal in hydrographic surveying.