Hydrographic surveys must be referenced to which type of control?

The correct answer focuses on the necessity for hydrographic surveys to be referenced to both horizontal and vertical control points. Horizontal control is essential for accurately determining the position of points on the Earth's surface, which is crucial for mapping underwater features and navigational routes. Vertical control is equally important as it establishes the elevation or depth, which allows for the accurate representation of underwater topography and changes in water levels.

In hydrographic surveying, the combination of these controls ensures that the data collected reflects the true position and depth of features relative to a standardized datum. This dual reference is vital for creating reliable charts and conducting safe navigation in maritime contexts. By using both horizontal and vertical controls, surveyors can produce detailed and accurate representations of aquatic environments that are critical for various marine applications, including navigation, resource management, and environmental studies.