Garmin Fishfinder 250 Owner's Manual

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Garmin Fishfinder 250 Owner's Manual Manual pdf 31 page image

23Understanding SonarIf you are unfamiliar with basic sonar, or need help determining what is displayed on the graph, thissection may be for you. This section is intended to help the novice user gain some understanding of how theFishfinder 250/250C 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 unitworks and how it determines what to display. Basically, the unit operates by transmitting sound wavestoward the bottom of a lake, stream, or seabed in a cone-shaped pattern. When a transmitted soundwavestrikes an underwater object such as the bottom, a piece of structure, or a fish, sound is reflected back to thetransducer. The transducer collects the reflected sound waves and sends the data to the unit to be processedand displayed on the graph. The underwater data is displayed on the graph in the order that it is returned:first returned—first on the graph. The diagram below demonstrates this by showing an underwater sceneas it would be displayed on the graph. Generally speaking, if the only thing between the transducer and thebottom is water, the first strong return will come from the bottom directly below the transducer. The firststrong return sets the bottom level. Weaker secondary returns provide the detailed data. The Fishfinder 250displays sonar returns as shades of gray; stronger returns are darker, and weaker returns are lighter. On theFishfinder 250C, sonar returns are displayed as red (strongest), then orange (strong), yellow (medium),green (weaker) and blue (weakest).The following pages will show how this data can help you to improve your fishing.This fish is currently in a dead zone and is notdetected by the sonar. The fish is in the coveragearea of the transducer, but remember– the firststrong return sets the bottom level. The fish willeventually be detected when the first strong returnsets the bottom level below the fish.On the WaterUnderstanding Sonar112233