Appendix G: Understanding Sonar - Garmin GPSMAP 178 Owner's Manual

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 178/178C operates and how it can help improve their fishing 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. Briefly 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 fish, sound is
reflected back to the transducer. The transducer collects the reflected 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: first returned—first on the chart. The diagram in the sidebar demonstrates 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 first strong return comes from the bottom directly
below the transducer. The first 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 fish where the strongest secondary returns are indicated
in black on the display.
That is brief description of how your GPSMAP 178/178C operates. Let' s take a look at how this data
can help you to improve your fishing.
Appendix G
Understanding Sonar
This fish is currently in a dead zone and is not detected by
the sonar. The fish is in the coverage area of the transducer,
but remember– the first strong return sets the bottom level.
The fish is eventually detected when the first strong return
sets the bottom level below the fish.