What is the datum used for a hydrographic survey?

In hydrographic surveys, the appropriate datum used is mean low water. This reference point is particularly significant because it provides a consistent baseline for measuring water depths and understanding navigational conditions in bodies of water. Mean low water represents the average depth of water at a low tide over a determined period, allowing for accurate assessments of underwater topography, the positioning of navigational hazards, and planning for maritime activities.

Using mean low water as a datum helps ensure that measurements are not influenced by fluctuating tidal conditions, leading to more reliable data for both navigation and marine construction. It also assists in achieving uniformity in surveying practices across different surveys and regions, making the information comparable and standard across various maritime jurisdictions.

In contrast, other options such as mean sea level or the high tide mark measure different water levels that can significantly vary due to tidal movements and other factors, thus making them less reliable for specifically hydrographic surveys focused on underwater features at low tide. Average river level could also prove inconsistent due to variable river flows influenced by weather patterns and seasonal changes. Therefore, mean low water is the optimal choice for ensuring precise hydrographic survey results.