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GPSMAP 496 Owner’s Manual 145
Setting up and Using Sonar > Understanding Sonar
Whiteline
The GSD 20 or GSD 21 Sounder Module can help you determine
whether the bottom is hard or soft. When the sonar sound waves are
reected 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 signicant 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 denition 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 496 to hear both weak and
strong signals simultaneously so as to identify sh returns under
the toughest conditions, such as suspended in thermoclines or even
hiding near structures. See-Thru technology allows the GSD 20 or
GSD 21 Sounder Module to see through thermoclines and helps
locate sh where they live.
Thermocline