Garmin GPSMAP 188C Owners Manual - Page 121

Appendix

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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 GPSMAP 188/188C/238 Sounder operates and how it can help improve their fishing 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. Briefly described, 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 fish, sound is reflected back to the transducer. The transducer collects the reflected 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: first returned-first on the chart. The diagram in the sidebar demonstrates this showing an underwater scene as it would be displayed on the chart. Generally speaking, if the only thing between the transducer and the bottom is water, the first strong return will come from the bottom directly below the transducer. The first 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 fish 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 fishing. Appendix I Understanding Sonar This fish is currently in a dead zone and is not detected by the sonar. The fish is in the coverage area of the transducer, but remember- the first strong return sets the bottom level. The fish will eventually be detected when the first strong return sets the bottom level below the fish. 111

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111
Appendix I
Understanding Sonar
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 fishing
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. Briefly 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 fish,
sound is reflected back to the transducer. The transducer collects the reflected 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: first returned—first 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 first strong return will come
from the bottom directly below the transducer. The first 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 fish 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 fishing.
This fish is currently in a dead zone and is not detected by
the sonar. The fish is in the coverage area of the transducer,
but remember– the first strong return sets the bottom level.
The fish will eventually be detected when the first strong
return sets the bottom level below the fish.