82 GPSMAP 292/298 Owner’s Manual
USING SONAR > UNDERSTANDING SONAR
Understanding the Sonar Screen
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.
Transducer Coverage
The area covered by the transmitted sound waves is determined
by the cone angle of the transducer and the water depth. The wide
cone angles (40°) associated with low frequencies (50 kHz) provide
a large coverage area for finding fish within a coverage width that
is approximately 2/3 of the water depth. However, this also means
that wide cone angles produce less bottom detail and resolution.
As shown in the drawing on the near right, the 40° cone angle
produces a coverage area of approximately a 20-foot diameter
circle at a 30-foot depth.
The narrow cone angles (10°) associated with the high frequencies
(200 kHz) provide better bottom resolution and crisper detail, but
cannot show a large coverage area for finding fish. The 10° cone
angle provides a coverage width that is approximately 2/10 of the
water depth. As shown in the drawing on the far right, the 10° cone
angle produces a coverage area of approximately a 6-foot diameter
circle at a 30-foot depth.
Dual frequency mode combines both frequencies to get the best
coverage area and contour/depth readings.
