Garmin GPSMAP 396 Owner's Manual - Page 152

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 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. 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 396 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 GSD 20 or GSD 21 Sounder Module to see through thermoclines and helps locate fish where they live. Thick Whiteline: Hard bottom Whiteline On Whiteline Off 144 Thermocline GPSMAP 396 Owner's Manual