Garmin GPSMAP 580 Owner's Manual page 81

Depth Number—allows you to select the efficiency of the
digital depth update rate. Fast Update updates quicker and is
recommended for low-noise water deeper than 50 feet. Auto is best
for shallow water or high-noise areas, has a slower screen update,
and is best used if you travel a wide variety of depths.
If the unit is unable to track the bottom for any reason, the digits in
the depth window flash on and off to alert you that the unit is not
tracking the bottom.
Map Split Sonar Detail—allows you to select how much detail
appears on the Map page with the sonar shown. Full Range shows
data using the Range setting of the Sonar page, regardless of Zoom
or Bottom Lock. Maximum Detail shows any Zoom or Bottom
Lock data from the Sonar page.
Keel Offset—allows you to enter the surface reading for the depth
of a keel. This makes it possible to measure depth from the bottom
of your keel rather than from the location of the transducer. Enter
a positive number to offset for a keel. It is also possible to enter
a negative number to compensate for a large vessel which draws
several feet of water. The Keel Offset is reflected in the depth
reading.
GPSMAP 580/585 Owner's Manual
Calibrating Water Speed
If you select Temp, Spd for the Transducer option on the Setup sub
tab, you must calibrate the water speed on your unit. The calibration
should take place in water with little or no current.
If available, the unit automatically uses the GPS ground speed, for
comparison when calibrating. GPS ground speed is not available,
use either your boat speedometer reading (not always accurate) or a
stopwatch to determine your speed over a certain distance. To find
out your speed, divide the distance by the time.
To calibrate water speed:
1. On the Setup sub tab, highlight the Calibrate Water Speed
button, and press Enter.
2. Bring the boat to cruising speed. Both the top ground and
uncalibrated water speeds are shown at the bottom of the
calibration window.
3. Note your top speed, then stop the boat, and press Enter.
NOTE: To calibrate water speed, you must have a transducer
with a built-in speed sensor or a separate compatible speed
sensor.
Setting Up and Using Sonar
75