Garmin 188c User Manual

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The transducer mounted on your vessel transmits sound waves 

toward the bottom of a lake, stream, or seabed in a cone-shaped 

pattern. When a transmitted sound wave 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 GPSMAP 

188/188C Sounder to be processed and displayed. The underwater 

data is shown on the Sonar display in the order that it is returned: 

first returned is the first on the display. 
Generally speaking, if the only thing between the transducer and 

the bottom is water, the first strong return comes from the bottom 

directly below the transducer and sets the bottom level. Weaker 

secondary returns provide the detailed data. Stronger returns 

display in darker colors, with red being the strongest return.

One of the first things to understand about sonar is that the 

transducer sends a beam down to the bottom of the water, much 

like the beam of a flashlight. The beam starts small near your 

vessel and expands as it gets to the bottom. Refer to “Transducer 

Coverage” for more detailed information.
The Sonar Page does not display a three-dimensional representation 

of the underwater environment; the display is in two-dimensions, 

much like if you took a picture of an aquarium. Only the depth of 

the item in the water is displayed. The Sonar display does not show 

you where an item is located horizontally in the water, as shown in 

the drawings below. The fish is not directly above the tree in reality, 

but it can look like they are on the Sonar Page.

20'

10'

1'

Fish

Tree

Tree