What is datum control in hydrographic surveying?

Datum control in hydrographic surveying is primarily concerned with establishing a reference framework for the measurements taken during surveys. It ensures that all data collected can be compared, interpreted, and utilized consistently over time and across different locations.

In this context, the correct answer emphasizes the importance of tidal datum control, which involves monitoring and maintaining the stability of tide gauge instrumentation. Since hydrographic surveys are heavily influenced by tidal movements, it is crucial to account for these variations to provide accurate depth measurements relative to a stable reference plane. This process involves not only the initial setup of instruments but also regular checks and calibrations to ensure that tide data remains reliable and consistent throughout the survey duration.

The other options, while related to the broader field of surveying, do not directly address the specific concept of datum control in a hydrographic context. The calibration of survey instruments and techniques for mapping underwater topography are aspects of survey practice, but they do not encapsulate the critical role that tide gauge stability plays in datum control. Similarly, measuring land elevations does not pertain directly to the context of hydrographic surveys, which focus on underwater data.