What do "sounding lines" measure?

Sounding lines are specifically designed to measure the depth of the water beneath a vessel. This is accomplished by lowering a weighted line overboard until it touches the bottom and then measuring the length of the line that has been submerged. This practice is essential for navigation, allowing mariners to determine safe water depths and avoid grounding in shallow areas. Knowing the depth is crucial for both safe navigation and operations that may require specific draft allowances for the vessel.

The other options pertain to different measurements: the speed of the ship's movement through the water is determined by devices such as logs or speed indicators, while pressure at various depths is typically measured with a pressure gauge or sensor, and water temperature is monitored using thermometers or temperature sensors. Each of these instruments serves a distinct purpose in maritime operations but does not relate to the measurement of depth, which is the primary function of sounding lines.