Section Three: On The Water; Understanding Sonar - Garmin Fishfinder 250 Owner's Manual

Understanding Sonar

If you are unfamiliar with basic sonar, or need help determining what is displayed on the graph, this
section may be for you. This section is intended to help the novice user gain some understanding of how the
Fishfi nder 250/250C operates and how it can help improve their fi shing productivity.
To understand what the unit is displaying, it is important to have a general knowledge of how the unit
works and how it determines what to display. Basically, the unit operates by transmitting sound waves
toward the bottom of a lake, stream, or seabed in a cone-shaped pattern. When a transmitted soundwave
strikes an underwater object such as the bottom, a piece of structure, or a fi sh, sound is refl ected back to the
transducer. The transducer collects the refl ected sound waves and sends the data to the unit to be processed
and displayed on the graph. The underwater data is displayed on the graph in the order that it is returned:
fi rst returned—fi rst on the graph. The diagram below demonstrates this by showing an underwater scene
as it would be displayed on the graph. Generally speaking, if the only thing between the transducer and the
bottom is water, the fi rst strong return will come from the bottom directly below the transducer. The fi rst
strong return sets the bottom level. Weaker secondary returns provide the detailed data. The Fishfi nder 250
displays sonar returns as shades of gray; stronger returns are darker, and weaker returns are lighter. On the
Fishfi nder 250C, sonar returns are displayed as red (strongest), then orange (strong), yellow (medium),
green (weaker) and blue (weakest).
The following pages will show how this data can help you to improve your fi shing.
This fi sh is currently in a dead zone and is not
detected by the sonar. The fi sh is in the coverage
area of the transducer, but remember– the fi rst
strong return sets the bottom level. The fi sh will
eventually be detected when the fi rst strong return
sets the bottom level below the fi sh.
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On the Water
Understanding Sonar
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