NorthStar Navigation NorthStar 8000i - Manual (Page 112)

112 Northstar 8000i Operation Manual
17 How to interpret the sounder screens
With practice and experience it's possible to interpret the echoes on the sounder
screens quite accurately. You can learn to recognize different fish, the type of
bottom, and shadows.
The water depth, the scroll speed and the echo strength can affect the quality of
the display. The range and gain settings can also affect the amount and quality of
echoes returned.
17.1 What are fish arches?
In good conditions, a fish swimming through the sonar cone will usually be shown
on the Sounder screen as a fish arch.
A fish arch starts when the fish first enters the edge of the sonar cone, generating
only a weak echo. This appears as the thin leading edge of the fish arch on your
Sounder screen.
As the fish swims towards your vessel and the center of the sonar cone, the echoes
are returned more strongly and more frequently. This means that the fish arch
increases in thickness and is shown at progressively shallower depths. This
produces the first half of the fish arch, which peaks as the fish passes through the
center of the sonar cone.
As the fish moves away from your vessel towards the edge of the sonar cone, the
reverse happens, producing the downturned side of the fish arch on your Sounder
screen.
17.2 Problems with fish arches?
There are several reasons why fish may not always appear as fish arches on your
Sounder screen. For example:
•
when you're at anchor, the fish appear as horizontal lines on the Sounder
screen as they swim in and out the sonar cone. A slow boat speed is more
likely to produce fish arches, especially in deep water
•
when you're in shallow water, the sonar cone is very narrow because it doesn't
have time to spread out, so the fish don't stay within it long enough to
generate a good fish arch on the display screen. Instead, the fish tend to
appear as randomly stacked blocks of pixels
•
when the water is choppy, the wave motion can distort the fish arches
•
the transducer may not installed in the best position
•
the range and range shift may need adjusting to show a smaller vertical extent
of water. Also try using the split zoom screen to increase the resolution. It can
be easier to see fish arches when the Sounder screen is concentrated on a
smaller area of water, rather than displaying everything from the surface to the
bottom.