Humminbird 141c - Manual (Page 61)

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Speed Through Water is provided by a speed paddlewheel, and is the measurement of
the flow past the boat, which may vary depending on current speed and direction.
Speed Through Water is most critical for anglers using downriggers, as it impacts the
running depth of the down riggers. Speed Over Ground is optimal for navigation, as
accurate destination times can be derived from this measurement. Humminbird®
products allow for input and display of both sources.
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 a
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.