GPSMAP 392/398/492/498 Owner’s Manual 83U SING SONAR > U NDERSTANDING SONARWhitelineWhen the sonar sound waves are reflected back by the bottom,a hard bottom returns a stronger signal than a soft bottom. Athin whiteline indicates a softer bottom, while a thick whitelineindicates a harder bottom. Normally, a red line is used to show thepoint where water meets the bottom. This line follows the bottomcontour, along with any significant objects lying on the bottom.The unit uses the whiteline function to make the bottom layerinformation easier to distinguish. These images show the ClassicBlack color scheme.Whiteline OffWhiteline OnThick whiteline:Hard bottomThermoclinesA rough definition of a thermocline is a break in water wherethe 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 392/398/492/498 to hearboth weak and strong signals simultaneously so as to identifyfish returns under the toughest conditions, such as suspended inthermoclines or even hiding near structures. See-Thru technologyallows the transducer to see through thermoclines and locate fish.This image is in the Classic Black color schemeThermocline
