Garmin GPSMAP 392, 398, 492, 498 Owner's Manual

Also see for GPSMAP 392, 398, 492, 498: Quick reference guide Specifications

Contents

82 GPSMAP 392/398/492/498 Owner’s ManualU SING SONAR > U NDERSTANDING SONAR• Scale—controls the appearance of the depth scale shownvertically along the right side of the graph. You can configurethe depth scale to appear in one of four different ways: as anOverlay, in the Corners, Basic, or No Scale.• Water Temperature Number and Water Speed Number—hides or shows temperature and water speed on the SonarPage. When set to Auto, the unit automatically shows thisinformation, only if it is received from the transducer.• Color Scheme—sets the colors for the Sonar and FlasherPages.NOTE: If the unit is unable to track the bottom for any reason,the digits in the depth window flash on and off to alert you thatthe unit is not tracking the bottom.Transducer CoverageThe area covered by the transmitted sound waves is determinedby the cone angle of the transducer and the water depth. The widecone angles (40°) associated with low frequencies (50 kHz/80kHz)provide a large coverage area for finding fish within a coveragewidth that is approximately 2/3 of the water depth. However, thisalso means that wide cone angles produce less bottom detail andresolution. As shown in the drawing on page 73, the 40° cone angleproduces a coverage area of approximately a 20-foot diametercircle at a 30-foot depth.The narrow cone angles (10°) associated with the high frequencies(200 kHz) provide better bottom resolution and crisper detail, butcannot show a large coverage area for finding fish. The 10° coneangle provides a coverage width that is approximately 2/10 of thewater depth. As shown in the drawing on the far right, the 10° coneangle produces a coverage area of approximately a 6-foot diametercircle at a 30-foot depth.Dual frequency or dual beam modes combine both frequencies toget the best coverage area and contour/depth readings.