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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 21 or GSD 20 Sounder Modules and then
to the GPSMAP 278 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
Generally speaking, 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 appear
in darker colors, with red being the strongest return.
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
GPSMAP 278 Owner’s Manual 99
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