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North, Magnetic: North is the principle reference point of the compass (i.e. North is 000°).
Magnetic North relies on the earth’s magnetic field to align a metal pointer generally towards
True North. However, due to local variations in the earth’s magnetic field around the globe, the
difference between True North and Magnetic North can be greater than 10°. See True North.
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.