Lowrance Mark-4 HDI Operation Manual page 31

Installation
Keel Offset
All transducers measure
water depth from
transducer to the bottom.
As a result, water depth
readings do not account
for the distance from the
transducer to the keel or
from the transducer to the
water surface.
Before setting keel offset,
measure the distance from
the transducer to the lowest part of the keel. If, for
example, the keel is 3.5 feet below the transducer,
it will be input as –3.5 feet.
Transducer
Keel Offset (-3.5 feet)
the
Installation menu
Keel
Water speed calibration
Calibrates a paddlewheel speed sensor with speed
data from a GPS source.
Temperature calibration
Calibrates data from the transducer temperature
sensor with data from a known temperature source
to ensure the accuracy of temperature information.
Reset water distance
Resets the Water Distance value to zero.
Transducer Type
Selects the transducer model attached
to your unit. You must select the
correct transducer type to get the best
sonar performance from your unit.
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Sonar Operation | Mark-4 & Elite-4 HDI