Understanding SonarThe transducer mounted on your vessel transmits sound wavestoward the bottom of a lake, stream, or seabed in a cone-shapedpattern. When a transmitted sound wave strikes an underwaterobject—such as the bottom, a piece of structure, or a fish—sound isreflected back to the transducer. The transducer collects the reflectedsound waves and sends the data to the GPSMAP 396 to be processedand shown. The underwater data is shown on the Sonar screen in theorder that it is returned: first returned is the first on the screen.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 screenin 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. Refer to “Transducer Coverage” formore detailed information.The Sonar Page does not show a three-dimensional representation ofthe underwater environment; the screen is in two-dimensions, muchlike if you took a picture of an aquarium. Only the depth of the itemin the water is shown. The Sonar Page does not show you where anitem is located horizontally in the water, as shown in the drawingsbelow. The fish is not directly above the tree in reality, but it canlook like it is on the Sonar Page.20'10'1'Aerial View of the Water Sonar PageFishTreeTreeGPSMAP 396 Pilot’s Guide 143SETTING UP AND USING SONAR > U NDERSTANDING SONAR
