42 GPSMAP 620 and GPSMAP 640 Owner’s ManualMarine Mode: Advanced ConfigurationSetting System AlarmsFrom the Home screen, touch Configure > Alarms > System.Clock—set an alarm using the system (GPS) clock. The unit mustbe on for the clock alarm to work.GpS Accuracy—set an alarm to sound when the GPS locationaccuracy falls outside the specified value.Setting Sonar AlarmsFrom the Home screen, touch Configure > Alarms > Sonar.Shallow Water/Deep Water—set an alarm to sound when thedepth according to a NMEA 0183 water-depth sensor is less than orgreater than the specified value.Water temp—set an alarm to sound when temperature receivedvia a NMEA 0183 sensor reports a temperature that is 2°F (1.1°C)above or below the specified temperature.Configuring My BoatFrom the Home screen, touch Configure > My Boat.Keel offset—offset the surface reading for the depth of a keel,making it possible to measure depth from the bottom of the keelinstead of from the transducer location. Enter a positive number tooffset for a keel. Enter a negative number to compensate for a largevessel that may draw several feet of water.To adjust the Keel Offset:1. From the Home screen, touch Configure > My Boat > KeelOffset.2. Use the on-screen keyboard to specify the offset.• If you are measuring down to the keel (transducer installedat the water line), enter a (+) positive number.• If you are measuring up to the water line (transducerinstalled on the keel), enter a (-) negative number.3. Touch Done to accept the number.
