Garmin gpsmap 178 User Manual

The area covered by the transmitted sound waves is determined by the cone angle of the transducer 

and the water depth. Cone angles may vary between different types of transducers. For example, a 50kHz 
frequency may provide a “wide” 40° cone angle, with a coverage width that is approximately 2/3 of the 
water depth. As shown in the sidebar, the 40° cone angle (50kHz frequency) approximately covers the 
area of a 20 foot diameter circle at a 30 foot depth. A 200kHz frequency may provide a “narrow” 10° 
cone angle, with a coverage width that is approximately 2/10 of the water depth. As shown, the 10° cone 
angle (200kHz frequency) approximately covers the area of a 6 foot diameter circle at a 30 foot depth. 

When using the GPSMAP 178/178C in ‘Dual’ frequency mode, the unit transmits both 50kHz and 

200kHz signals at the same time. The ‘Dual’ frequency capability of the GPSMAP 178/178C allows you 
to have a large coverage area and still retain good bottom resolution. When in ‘Dual’ frequency mode the 
unit uses the narrow cone angle (200kHz) to display detailed bottom information, keeping “Dead Zones” 
to a minimum, and the wide cone angle (50kHz) for the large coverage area.

Fish returns from the wide and narrow beams appears differently on the display. The wide beam 

tends to show longer fish returns than the narrow beam.

200kHz - 10˚ Cone Angle

72" coverage at 30'

50kHz - 40˚ Cone Angle

20' coverage at 30'