What is the reference used for depth measurements on nautical charts?

What is the reference used for depth measurements on nautical charts?

• A. Mean Low Water
• B. Mean High Water
• C. Chart Datum
• D. Lowest Astronomical Tide

Answer: (A)

Depth measurements on nautical charts are referenced to a fixed zero level called Chart Datum. This datum is set by the hydrographic authority and serves as the consistent baseline from which all soundings are measured. It is not the same as the instantaneous tide at a location, which is why chart depths must be adjusted using the current tide when calculating actual water depth. In many regions, Chart Datum is defined as Lowest Astronomical Tide, but the essential idea is that chart depths are tied to Chart Datum, a stable reference, rather than to Mean Low Water or Mean High Water, which describe typical tidal levels rather than the chart’s depth zero.

Depth measurements on nautical charts are referenced to a fixed zero level called Chart Datum. This datum is set by the hydrographic authority and serves as the consistent baseline from which all soundings are measured. It is not the same as the instantaneous tide at a location, which is why chart depths must be adjusted using the current tide when calculating actual water depth. In many regions, Chart Datum is defined as Lowest Astronomical Tide, but the essential idea is that chart depths are tied to Chart Datum, a stable reference, rather than to Mean Low Water or Mean High Water, which describe typical tidal levels rather than the chart’s depth zero.