Garmin GPSMAP 238 Sounder User Manual download pdf (Page 121)

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Appendix I

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.

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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Garmin GPSMAP 238 Sounder User Manual Page 121