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What Is a Depth Finder?

Sonar determines depth and the location of fish.
A commercial fishing boat using a depth finder.
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  • Written By: Daniel Liden
  • Edited By: Jenn Walker
  • Last Modified Date: 23 September 2014
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A depth finder is a device used on many boats to determine the depth of the water and the depth of objects in the water at a given point. They are used for many purposes, such as determining where it is deep enough to safely move a boat and determining the depth and location of fish. Many fishermen use fish depth finders that can often pinpoint the precise location and size of a fish near the boat. They can often provide other information about the bottom of the body of water, such as the amount of aquatic foliage and the degree of rockiness.

Sonar, which stands for sound navigation and ranging, is the method typically used by a depth finder or fish depth finder to determine depth. Sound waves are sent down through the water; they bounce back to the sonar device when they strike an object such as a fish or the bottom of a lake. Based on the time it takes for the wave to be reflected and on the size of the deflected wave, the device can produce a digital representation of what exists under the water. Some devices simply report the depth, while others show precise three dimensional diagrams that indicate the depth and locations of fish and other features beneath the water.

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A depth finder can be a very important navigational tool. Some boats, especially large ones, extend deep under the water; this is often particularly the case with boats that have large external propellers. The operators of such boats need to ensure they do not strike the bottom, damaging the vessel. A depth finder can indicate the depth, allowing one to steer clear of shallow areas that could be potentially hazardous.

Both personal and commercial fishermen often make use of fish depth finders to locate fish. They can be used in many different bodies of water, from small lakes to the ocean. Usually, an expensive commercial fish depth finder works at greater depths and with greater precision than a less expensive personal fish depth finder. Such devices can sometimes mistake aquatic foliage and stones for fish, though they can usually locate fish with a fair amount of precision.

A depth finder is typically made up of two parts. A screen allowing for data input and viewing is placed on the boat, while a sonar device that emits and interprets sound waves is placed beneath the boat. The viewing device can display information with varying degrees of detail, from small blips of light to three dimensional maps of the bottom of the body of water. More detailed depth finders typically cost significantly more money.

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