Furuno GP-1850WF - Manual (Page 27)

2. VIDEO SOUNDER OPERATION
2-11
2.15 Interpreting the Display
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.
Zero
line
Shift
Zero line
Minute mark
Minute mark shows time with two colored bar.
Each bar is 30 sec.
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.
Small
school
Large
school
Size of
fish school
Fish school echoes
Bottom echo
Echoes from the bottom are normally the
strongest and are displayed in reddish-brown
color 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.
Intensity
difference in
water depth
Second bottom
echo
Rock base
Mud
and sand
Bottom echoes