How is water depth defined in a hydrographic survey?

In hydrographic surveying, water depth is defined as a measurement from the mean lower low water level to the seafloor. This standardization is crucial because it provides a consistent reference point for measuring depth, which is particularly important for navigation, marine construction, and resource management. By measuring from the mean lower low water level, surveyors can account for varying tidal conditions, ensuring that depth measurements are reliable and meaningful.

Using mean lower low water as a baseline helps avoid discrepancies that could arise from changes in water level due to tidal influences. This practice is essential for safe navigation, as it allows mariners to understand the minimum depth they can expect under the most extreme tidal conditions, thus ensuring that vessels can safely pass without running aground.

In contrast, other definitions like the distance from the water surface to the seabed don't take into account tidal variations, which can lead to misleading measurements depending on the time of day. Additionally, measuring between two underwater features does not provide an accurate representation of the depth of the water column, nor does defining the depth of the ocean floor at low tide capture the dynamic nature of tides and their impact on water levels.