Humminbird 1158c Combo Installation And Operation Manual page 227

Glossary Sonar
Target Separation
The measurement of minimum distance that a fishfinder needs to be able to recognize
two very close objects as two distinct targets (i.e. two fish hanging very close, or a fish hanging very close
to structure). Humminbird® fishfinders provide a very good 2 1/2 inches of target separation in shallower
than 100 feet of depth. Target separation decreases as depth increases due to the need for longer Pulse
Pulse Width
Width to achieve greater depth. See .
Thermoclines
Water layer(s) of distinctly different temperatures that create a sonar reflection due to the
density of the differing water temperatures. Typically a thermocline will appear as a continuous band across
the display at some distance above the bottom contour. Thermoclines are of interest to anglers because fish
will suspend above or below the thermocline as they seek the optimum temperature and oxygen levels.
Time Variable Gain (TVG)
A processing step applied to the sonar return to "normalize" the data so that
objects of equal size (i.e. fish) appear to be the same size, even if they are separated by a good distance.
Transducer
The transducer is part of the sonar system, which mounts on the boat and is in contact with
the water, that converts the electrical energy from the transmitter into sound energy, and that forms the
sonar beam in turn. Internally, the transducer consists of one or more piezo electric disks that expand by very
minute amounts to create the sound wave. This element also works in reverse, converting the returned
sound energy back into an electrical signal that the receiver interprets. See SONAR.
Zoom
A feature that focuses on a smaller area of the bottom to provide enhanced resolution. With
enhanced resolution, the angler can more easily see fish hanging in structure or multiple fish hanging close
together.
Zoom, Bottom Lock
Bottom Lock Zoom is a feature that focuses on a smaller area just above the bottom
to provide enhanced resolution. Unlike regular zoom, it continuously graphs the bottom at a constant point
on the display regardless of changes in depth. This "flattens" out the bottom contour, but is effective at
showing fish on or near the bottom, and is preferred by many saltwater anglers.
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Glossary - Sonar