If the height of the object is known, the stadimeter may be used. It operates on the principle that the closer an object is, the bigger it appears to be. In shallow water, soundings help fix a ship's position. Sonic, or echo, depth finders make use of the known speed of sound in water. Sound transmitted from the ship is reflected from the ocean floor to a receiver, which measures elapsed time and calculates distance.
Some devices produce fathograms continuous profiles, or graphs, of the ocean bottom.
An older depth-finding device is the hand lead and line a marked cord with a weight on the end. The length of time required for a light to complete a full cycle of changes is called the period of the light. United States waters are marked for safe navigation by the lateral system of buoyage . Simple arrangements of colors, shapes, numbers, and lights are employed to indicate the side of a buoy on which a ship should pass when moving in a given direction. In dead reckoning, the navigator estimates a ship's position by keeping a careful record of its movement.