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A depth sounder is an electronic device which measures the depth of the water a boat is traveling in. Most depth sounder designs allow for an alarm to go off at a predetermined water depth. This ensures that the boat does not strike bottom and become beached. It also ensures that an engine or propeller does not become damaged from hitting bottom. Many upscale depth sounder designs have provisions to set certain areas into the Global Positioning System (GPS) so that the ship can be forewarned on its next voyage to the same location.
Prior to the invention of the depth sounder, ships used to troll heavy objects and take measurements from weights tied to long ropes. The weight could be lowered and the rope measured when the weight touched the bottom. Often, this slow process found the ship becoming beached or damaged before the measurement could be completed. Modern electronic depth sounder devices have real-time feeds and measure to the exact depth instantly.
The depth sounder not only gives an accurate measurement of the water's depth—most units give an accurate picture of the bottom contour and makeup. The device can show the contour of rocks or sunken ships or humps and dips by using radar. The depth sounder uses a radar beam to scan the bottom and receive the radar beams as they are bounced back up off of the bottom. The radar not only allows the operator to "see" the depth of the water, but also the slope of the drop-off or any objects which may be taller than the bottom and could damage the ship even though the ship is in deep enough water.
When entering many ports, some of today's super-sized ships miss the bottom of the harbor by a very narrow margin. Often, the difference of only a minuscule amount will determine success or danger for a ship. Depth sounders allow the ship's crew to accurately navigate the shallow areas and avoid any situation which might cause the ship to be in peril—for these ships, the sounder's ability to not only monitor below the vessel but to scan each side of the vessel aids in navigating the channels. For smaller pleasure or sporting craft, the depth sounder allows safe navigation when entering a non-familiar area for the first time or even at night. When the captain hears the sounder alarm go off, it is time to drop anchor or to back slowly away from the shallow area.