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GPSMAP 188/188C Sounder Owner’s Manual 87
USING SONAR > SONAR TAB OPTIONS
Keel Offset—used to offset the surface reading for the depth of
a keel. This makes it possible to measure depth from the bottom
of your keel instead of from the transducer’s location. Enter a
positive number to offset for a keel, or enter a negative number to
compensate for a large vessel that can draw several feet of water.
The Keel Offset reflects in the depth reading.
Keel Offset:
Transducer at
Surface Enter (+)
positive number to
show depth from
bottom of keel
Keel Offset: Transducer
at Bottom of Keel Enter
(-) negative number to
show depth from surface
Map Split Sonar Detail—controls Map Page sonar split range
scale. Full Range shows the entire scale of 2X or 4X left side split
of Sonar Page. Maximum Detail displays what is shown on the left
side of a split sonar screen.
Background Color—sets the background color on the Sonar Page
to Black, Blue, or White.
Calibrate Water Speed—this is only available if you are using
a speed-capable transducer/sensor. Calibration is required to
ensure the Water Speed displayed on your unit is accurate. The
unit automatically use the GPS ground speed, if it is available,
for comparison on the calibration. If a GPS ground speed is not
available, use either your boat’s speedometer reading (not always
accurate) or a stopwatch to determine your speed over a certain
distance (distance / time = speed). It is recommended that the
calibration take place in water having little or no current.
Calibrating the
Speed Sensor