Northstar
FISH 457/467
Installation and Operation Manual
17
3-2 Interpreting the display
The sonar displays do not show a fixed distance
travelled by the boat; rather, they display a
history, showing what has passed below the boat
during a certain period of time.
The history of the sonar signal displayed depends
the depth of the water and the scroll speed
setting.
In shallow water, the echoes have a short distance
to travel between the bottom and the boat. In
deep water, the history moves across the display
more slowly because the echoes take longer to
travel between the bottom and the boat. For
example, when the scroll speed is set to
Fast
,
at depths over 1000 ft (300 m) it takes about 6
minutes for the data to move across the display,
whereas at 20 ft (6 m) it takes only about 10
seconds.
The scroll speed can be set by the user to display
either a longer history with less fish information
or a shorter history with more fish details (see
section 8-2).
If the boat is anchored, the echoes all come from
the same area of bottom. This produces a flat
bottom trace on the display.
The screen shot shows a typical sonar display
with the Fish symbols turned
Off
.
Strength of echoes
The colors indicate differences in the strength of
the echo. The strength varies with several factors,
such as the:
•
Size of the fish, school of fish or other object.
•
Depth of the fish or object.
•
Location of the fish or object. (The area
covered by the ultrasonic pulse is a rough
cone shape and the echoes are strongest in
the middle.)
•
Clarity of water. Particles or air in the water
reduce the strength of the echo.
•
Composition or density of the object or
bottom.
CAUTION
Note:
Planing hulls at speed produce air
bubbles and turbulent water that bombard the
transducer. The resulting ultrasonic noise may
be picked up by the transducer and obscure
the real echoes.
A
C
D
B
A Single
fish
B Large school of fish
C Small school of fish
D Bottom
A Kelp / Weed
B Soft bottoms such as mud, weed and sand
show as narrow bands
C Hard bottoms such as rock or coral show as
wide bands
A
C
B
