Definition
The instrument measures from the transducer face. Setting a positive offset equal to the transducer's depth below the waterline gives depth below the surface, which is what compares directly with charted depth plus height of tide. Setting a negative offset equal to the distance from transducer to the lowest point of the keel gives clearance under the keel. Whichever is chosen must be known by everyone aboard, since confusion of the two has grounded many yachts.
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