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GPSMAP 188/188C Sounder Owner’s Manual 89
USING SONAR > SONAR TAB OPTIONS
Understanding Sonar
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.
Understanding the Sonar Display
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'
Aerial View of the Water Sonar Display
Fish
Tree
Tree