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GPSMAP 198C 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 198C
Sounder to process and show. The underwater data is shown on the
Sonar Page in the order that it is returned: first returned is the first
on the screen.
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 show
in darker colors, with red being the strongest return.
Understanding the Sonar Screen
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 show a three-dimensional representation
of the underwater environment; the screen is in two-dimensions,
much like if you took a picture of an aquarium. Only the depth of
the item in the water is shown. The Sonar screen 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 it is on the Sonar Page.
20'
10'
1'
Aerial View of the Water Sonar Page
Fish
Tree
Tree