Garmin GPSMAP 198C Owner's Manual page 97

Whiteline
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.
Thick
Whiteline: Hard
bottom
Whiteline On
GPSMAP 198C Sounder Owner's Manual
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 198C Sounder to hear
both weak and strong signals simultaneously so as to identify
fish returns under the toughest conditions, such as suspended in
thermoclines or even hiding near structures. See-Thru technology
allows the transducer to see through thermoclines and locate fish.
Whiteline Off
U S > U
SING ONAR NDERSTANDING
®
Thermocline
S
ONAR
91