Filter
Top Products
111
Appendix G
Understanding Sonar
Understanding Sonar
If you are unfamiliar with basic sonar, or need help determining what is displayed on the graph,
this section may be for you. This section is intended to help the novice user gain some understanding
of how the GPSMAP 178C operates and how it can help improve their fi shing productivity.
To understand what the unit is displaying, it is important to have a general knowledge of how the
unit works and how it determines what to display. Briefl y described, the unit operates by transmitting
sound waves toward the bottom of a lake, stream or seabed in a cone shaped pattern. When a transmit-
ted soundwave strikes an underwater object such as the bottom, a piece of structure, or a fi sh, sound is
refl ected back to the transducer. The transducer collects the refl ected sound waves and sends the data
to the unit to be processed and displayed on the chart. The underwater data is displayed on the chart
in the order that it is returned: fi rst returned—fi rst on the chart. The diagram in the sidebar demon-
strates this showing an underwater scene as it would be displayed on the chart. Generally speaking, if
the only thing between the transducer and the bottom is water, the fi rst strong return comes from the
bottom directly below the transducer. The fi rst strong return sets the bottom level. Weaker secondary
returns provide the detailed data. Stronger returns displays in darker colors, black being the strongest
return. In Example 3 (page 113) you can see that the branches and the fi sh where the strongest second-
ary returns are indicated in black on the display.
That is brief description of how your GPSMAP 178C operates. Let’s take a look at how this data can
help you to improve your fi shing.
This fi sh is currently in a dead zone and is not detected by
the sonar. The fi sh is in the coverage area of the transducer,
but remember– the fi rst strong return sets the bottom level.
The fi sh is eventually detected when the fi rst strong return
sets the bottom level below the fi sh.
GPSMAP178CManual.indd 111 3/19/2004, 3:11:10 PM