Humminbird 757c - Manual (Page 159)

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Structure: Structure is a general term for objects on the bottom that present a
discontinuity and are a likely attractor for fish. This includes bottom contour
features (drop-offs, humps, and holes), standing structure (stumps, timbers, brush
piles) and a wide range of other potential objects (sunken boats, reefs).
Humminbird® units excel at showing structure with great detail over a wider area
due to unique sonar configurations developed for the angler.
Structure ID®: Structure ID® is a Humminbird® feature that describes the
traditional grayscale method of presenting sonar information. See Grayscale.
Surface Clutter: Surface Clutter is a phenomenon where sonar returns are
reflected off of tiny objects near the surface of the water, including algae and even
air bubbles. Typically, saltwater environments have significantly greater surface
clutter than freshwater due to continuous wind and wave action that causes
aeration at the surface. The Surface Clutter menu provides manual control to bias
the default settings under extreme conditions.
Target Separation: Target Separation is 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 Width to achieve greater depth. See Pulse Width.
Thermoclines: Thermoclines are 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: Time Variable Gain is 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. Time Variable Gain
is a fundamental attribute of good sonar, but is often promoted as a feature.
Total Screen Update®: A Humminbird® feature that refreshes and updates all the
sonar information on the display when a range change occurs. Without Total
Screen Update®, only the most recent sonar information would be drawn to the
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