Owner's Manual
Understanding Sonar
The transducer mounted on your vessel transmits sound waves
toward the bottom of the water in a cone-shaped pattern. When
a transmitted sound wave strikes an underwater object—such as
the bottom, a structure, or a sh—sound is reected back to the
transducer. The transducer collects the reected sound waves and
sends the data to the GSD 20 or GSD 21 Sounder Module and then
to the GPSMAP 276C to be processed and shown. The underwater
data is shown on the Sonar display in the order that it is returned;
rst returned is the rst on the display.
Sonar Page
If the only thing between the transducer and the bottom is water,
the rst 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.
Understanding the Sonar Display
One of the rst 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 ashlight. The beam starts small near your vessel and
expands as it gets to the bottom. See “Transducer Coverage” for
more detailed information.
The Sonar Page does not show 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 appears. The Sonar display does not show you where an
item is located horizontally in the water, as shown in the drawings
below. The sh is not directly above the tree in reality, but it might
look like it is on the Sonar Page.
20'
10'
1'
Aerial View of the Water Sonar Display
Fish
Tree
Tree
98 GPSMAP 276C Owner’s Manual
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