Garmin Marine GPS System GPSMAP User Manual

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Whiteline
The GSD 21 Sounder Module can help you determine whether the
bottom is hard or soft. When the sonar sound waves are reected
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 signicant objects lying on the
bottom. The unit uses the whiteline function to make the bottom
layer information easier to distinguish.
Whiteline OffWhiteline On
Thick
Whiteline: Hard
bottom
Thermoclines
A rough denition 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 378/478 to hear both weak
and strong signals simultaneously so it can identify sh returns
under the toughest conditions, such as suspended in thermoclines or
even hiding near structures. See-Thru
®
technology allows the
GSD 21 to “see” through thermoclines and helps locate sh where
they live.
Thermocline
114 GPSMAP 378/478 Owner’s Manual
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