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3. INTERPRETING THE DISPLAY
3.1 Zero Line
The zero line (sometimes referred to as the transmission line)
represents the transducer`s position, and moves off the screen
when a deep phased range is used.
3.2 Fish School Echoes
Fish school echoes will generally be plotted between the zero
line and the bottom. Usually the fish school/fish echo is
weaker than the bottom echo because its reflection property is
much smaller compared to the bottom. The size of the fish
school can be ascertained from the density of the display.
3.3 Bottom Echo
Echoes from the bottom are normally the strongest and
are displayed in reddish-brown color (in default color
arrangement) but the color and width will vary with
bottom composition, water depth, frequency, sensitivity,
etc.
In a comparatively shallow depth, a high gain setting will
cause a second or sometimes a third or a fourth echo to
be displayed at the same interval between them below
the first echo trace. This is because the echo travels
between the bottom and the surface twice or more in
shallow depths.
The color of the bottom echo can be used to help
determine the density of the bottom materials (soft or
hard). The harder the bottom, the wider the trace. If the
gain is set to show only a single bottom echo on mud, a
rocky bottom will show a second or third bottom return.
The range should be chosen so the first and second
bottom echoes are displayed when bottom hardness is
being determined.
Zero
line
Shift
Size of
fish school
Intensity
difference in
water depth
Second bottom
echo
Rock base
Mud
and sand
3-1