Understanding SonarThe transducer mounted on your vessel transmits sound wavestoward the bottom of the water in a cone-shaped pattern. Whena transmitted sound wave strikes an underwater object—such asthe bottom, a structure, or a fish—sound is reflected back to thetransducer. The transducer collects the reflected sound waves andsends the data to the GSD 20 Sounder Module and then to theGPSMAP 376C to be processed and displayed. The underwater datais shown on the Sonar display in the order that it is returned; firstreturned is the first on the display.Sonar PageGenerally speaking, if the only thing between the transducer andthe bottom is water, the first strong return comes from the bottomdirectly below the transducer and sets the bottom level. Weakersecondary returns provide the detailed data. Stronger returns displayin darker colors, with red being the strongest return.Understanding the Sonar DisplayOne of the first things to understand about sonar is that thetransducer sends a beam down to the bottom of the water, much likethe beam of a flashlight. The beam starts small near your vessel andexpands as it gets to the bottom. See “Transducer Coverage” formore detailed information.The Sonar Page does not display a three-dimensional representationof the underwater environment; the display is in two-dimensions,much like if you took a picture of an aquarium. Only the depth ofthe item in the water appears. The Sonar display does not show youwhere an item is located horizontally in the water, as shown in thedrawings below. The fish is not directly above the tree in reality, butit might look like it is on the Sonar Page.20'10'1'Aerial View of the Water Sonar DisplayFishTreeTree112 GPSMAP 376C Owner’s ManualSETTING UP AND USING SONAR > U NDERSTANDING SONAR
