Furuno SFD-1010 Operator's Manual page 126

3. FISH FINDER
Size of a school of fish
If two fish echoes appear at different
depths with the same size, the school of
fish at the shallower depth is actually
larger, which you should be cautious of.
This is because the conical ultrasonic
beam propagates longer to a school of
fish in deep water and thus the school of
fish is displayed longer. The size of fish
echoes also varies with the own ship's
speed and picture advance speed
settings.
Display colors of a school of fish
A school of fish of the same size but with a higher density gives a stronger reflection
signal. The ultrasonic wave attenuates as it propagates deeper and the school of fish
in deep water is displayed in a weaker color than those of the same density in shallow
water. This means that if two schools appear with the same display color in shallow
water and deep water, the one in deeper water is actually denser. By constantly
comparing actual fish volume against these references in each case, you will be able
to analyze the schools of fish more accurately. Raising the clutter value sequentially
erases colors from weaker one, such as for plankton, so that the schools of fish can
be recognized more easily.
Bottom fish discrimination
The echoes from the bottom are stronger than
those from the schools of fish. So you can
distinguish between them by display colors.
With proper gain calibration, the bottom echoes
are shown in reddish-brown or red and the
school of fish echoes are shown in weaker
color.
3-34
Less reddish Reddish
(Sparse echo) (Dense echo)
Fish echo
Difference in fish school depth and display time
Display time for a shallow school of fish
Display time for a deep school of fish
Difference in signal strength
Signal is
attenuated
Strong signal
is returned