A fishfinder is a type of Fathometer, both being specialized types of echo sounding systems, a type of Active SONAR. ('Sounding' is the measurement of water depth, a historical nautical term of very long usage. more...
) The fishfinder uses active sonar to detect fish and 'the bottom' and displays them on a graphical display device, generally a LCD or CRT screen. In contrast, The modern fathometer (from fathom plus meter, as in 'to measure') is designed specifically to show depth, so may use only a digital display (useless for fish finding) instead of a graphical display, and frequently will have some means of making a permanent recording of soundings (which are merely shown and subsequently electronically discarded in common sporting fishfinder technology) and are always principally instruments of navigation and safety. The distinction is in their main purpose and hence in the features given the system. Both work the same way, and use similar frequencies, and, display type permitting, both can show fish and the bottom. Thus today, both have merged, especially with the advent of computer interfaced multipurpose fishfinders combining GPS technology, digital chart-plotting, perhaps Radar and electronic compass displays in the same affordable sporting unit.
In a generalized sense, an electrical impulse from a transmitter is converted into a sound wave by the transducer, called a hydrophone, and sent into the water. When the wave strikes something such as a fish, it is reflected back and displays size, composition, and shape of the object. The exact extent of what can be discerned depends on the frequency and power of the pulse transmitted. The signal is quickly amplified and sent to the display. Knowing that the average speed of the wave in the water is 4800 ft/s (1500 m/s) in seawater, 4708 ft/s (1435 m/s) in freshwater, both at normalized temperatures, the distance to the object that reflected the wave can be determined. The process can be repeated up to 40 times per second and eventually results in the bottom of the ocean being displayed versus time (the fathometer function that eventually spawned the sporting use of fishfinding.) Note: This discussion of the propagation of sound in water is simplified, speed of sound in water depends on the temperature, salinity and ambient pressure (depth). This follows approximately this formula (del Grosso, 1974):
c = 1448.6 + 4.618T − 0.0523T2 + 1.25 * (S − 35) + 0.017D
c = sound speed (m/s)
T = temperature (degrees Celsius)
S = salinity (pro mille)
D = depth
This will give variations in speed through the water column
The image above, at right, clearly shows the bottom structure -- plants, sediments and hard bottom are descernible on sonar plots of sufficiently high power and appropriate frequency. Slightly more than halfway up from the bottom to the left of the screen center and about a third away from the left side, this image is also displaying a fish -- a light spot just to the right of a 'glare' splash from the camera's flashbulb. The X-axis of the image represents time, oldest (and behind the soundhead) to the left, most recent bottom (and current location) on the right; thus the fish is now well behind the transducer, and the vessel is now passing over a dip in the ocean floor or has just left it behind. This obviously depends on both the speed of the vessel and how often the image is updated by the echo sounder.
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