Appendix I: Understanding Sonar - Garmin GPSMAP 188 Owner's Manual And Reference 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 understand-
ing of how the GPSMAP 188/188C/238 Sounder 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. Briefl y described, the unit operates by transmit-
ting 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 chart. The underwater data is displayed on
the chart in the order that it is returned: fi rst returned—fi rst on the chart. The diagram in the sidebar
demonstrates this showing an underwater scene as it would be displayed on the chart. Generally speak-
ing, 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. Stronger returns displays in darker colors, black being
the strongest return. In Example 3 (page 113) you can see that the branches and the fi sh where the
strongest secondary returns are indicated in black on the display.
That is brief description of how your GPSMAP 188/188C/238 Sounder operates. Let' s take a look at
how this data can help you to improve your fi shing.
Appendix I
Understanding Sonar
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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