Humminbird 161 - Manual (Page 107)

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North, True: North is the principle reference point of the compass (i.e. North is 000°).
True North uses the axis of the rotation of the earth as the reference for North and
is constant for all points on the earth. Most GPS units default to True North as the
setting. The GPS heading may vary from the boat compass heading because the
compass uses Magnetic North. Humminbird® GPS units provide the option of
selecting True or Magnetic readouts by using the North Reference menu. See
Magnetic North.
Present Position: Present Position is the current location (latitude, longitude)
determined by GPS. The Present Position is indicated on-screen by a directional boat
icon if the boat is moving, or a round symbol if the boat is stationary.
Route: A Route is series of waypoints linked together in a specific sequence to define a
navigation path between two points, and saved in the fishfinder’s memory. Routes are
often used when frequently traveling between two or more locations because it is a faster
and more accurate method than repeatedly selecting individual waypoints each time one
travels through the same area. Routes consist of a Start Waypoint, an End Waypoint, and a
series of waypoints in between. Users can begin navigation at the beginning, the end (to
run the route in reverse) or at any waypoint in between. See Waypoint.
SD: SD is an acronym for Secure Digital. An SD card is virtually identical in function
to an MMC; however, the speed at which information can be accessed is generally
faster, the storage capacity is generally larger, and the card has security features
which can be used to prevent extracting the data from the card. SD is the same
format that many digital cameras use. See MMC.
Selective Availability: Selective Availability (SA) is a GPS system “feature” that can be
implemented by the U.S. Department of Defense (DoD) to degrade the accuracy of GPS
positioning information. With SA active, the GPS is accurate to within 100 meters 95%
of the time. With SA inactive, the GPS is accurate to within 45 meters 95% of the time.
Currently, SA is inactive and the consumer can enjoy the greater precision GPS
provides; however, the DoD reserves the right to activate SA at any time.
Speed Over Ground (SOG): Speed Over Ground is the measurement of the boat’s
progress across a given distance, and is the speed measurement provided by GPS. An
alternate, Speed Through Water, is provided by a speed paddlewheel, and is the
measurement of the flow past the boat, which may vary depending on current
speed and direction. Speed Over Ground is optimal for navigation because accurate
destination times can be derived from this measurement. Humminbird® products
allow for input and display of both speed measurements.