Garmin GPSMAP 276C Owner's Manual page 108

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Whiteline
The GSD 20 or GSD 21 can help you determine whether the bottom
is hard or soft. When the sonar sound waves are reflected back
by the bottom, a hard bottom returns a stronger signal than a soft
bottom. A thin whiteline indicates a softer bottom, while a thick
whiteline indicates a harder bottom. Normally, a red line is used
to show the point where water meets the bottom. This line follows
the bottom contour, along with any significant objects lying on the
bottom. The unit uses the whiteline function to make the bottom
layer information easier to distinguish.
Thck
Whtelne: Hard
bottom
Whiteline On
100
u s
nderstAndIng onAr
Whiteline Off
Thermoclines
A rough definition of a thermocline is a break in water where
the water temperature changes faster than the water above it.
Thermoclines are shown as the weakest colors.
One of the unique features offered by Garmin is See-Thru
technology, which allows the GPSMAP 276C to hear both weak and
strong signals simultaneously so it can identify fish returns under
the toughest conditions, such as suspended in thermoclines or even
hiding near structures. See-Thru
GSD 21 to "see" through thermoclines and helps locate fish where
they live.
®
technology allows the GSD 20 or
®
Thermoclne
GPSMAP 276C Owner's Manual