Navman FISH 4200 Installation And Operation Manual page 10

The following picture shows the HISTORY screen with
the fish symbols turned off:
Strength of returned echoes
The shading indicates differences in the strength of
the returned echo from the bottom. A black pattern
indicates a strong echo, and light grey indicates a weak
echo. Fish symbols are always shown in black.
Shadows
Shadows are areas where the ultrasonic beam cannot 'see'. These
include hollows on the bottom or beside rocks and ledges where the
strong echo returned off the rock obscures the weaker echo of the fish.
When looking for small objects including fish, users should consider
that there are places where fishfinders cannot 'see'.
Shadow - where objects are hidden in the bottom echo.
10
Single fish
Large school of fish
Small school of fish
Bottom
These fish will be hidden in the bottom echo.
The strength of the returned echo varies with several
factors, such as:
· The size of the fish, school of fish or other
object.
· The depth of the fish or object.
· The location of the fish or object. The area
covered by the ultrasonic beam is
approximately cone-shaped and echoes are
strongest in the middle.
· The clarity of water. Particles or air in the water
reduce the strength of the returned echo.
· The composition or density of the object or
bottom. Mud, weed and sand bottoms tend to
weaken and scatter the sonar signal, which
results in weaker returns. Rock or coral
bottoms concentrate the return signal for
strong returns.
Note: that 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.
Large fish
Kelp / weed
Hard bottoms such as
rock or coral show as
wide bands at the
bottom of the screen
Soft bottoms such as
mud, weed and sand
show as narrow bands
at the bottom of the
screen
FISH 4200 Installation and Operation Manual