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such as that included with many Humminbird® products, receives the signals from
these satellites and is able to determine position based on very slight differences in the
time each signal is received and the receiver’s knowledge of the location of each of the
satellites.
Heading: Heading describes the direction the boat is pointing and is measured in
degrees (i.e. 321°, where 000° is North, 090° East, 180° is South, 270° is West). Due to
wind and waves, the boat is often traveling in a slightly different direction than it is
heading, and therefore Heading is often confused with Course Over Ground. See Course
Over Ground.
Map Datum: A Map Datum refers to a particular survey of the earth’s surface that was
referenced when creating a chart (i.e. WGS84). Since the earth is not flat, and not even
consistently round, a mathematical model must be used to translate coordinates from
the sphere of the earth to the flat surface of a chart. This mathematical model is the
Map Datum. Historically, many different Map Datums have been used around the world
to create maps; often a reference to the Map Datum can be found in the legend of a
paper map. It is important to select the correct Map Datum setting in your fishfinder
when comparing GPS position to a paper chart to avoid slight position inaccuracies.
Almost all electronic charts use the WGS84 Map Datum and therefore, they do not
require setting changes.
MMC: MMC is an acronym for Multi Media Card. An MMC is a postage stamp-sized
memory device used to store electronic data such a maps, waypoints, routes and other
information. MMC is a very rugged format suitable for the marine environment, but it is
not waterproof. The MMC is removable from Humminbird® products, and can be used
in a PC that is equipped with an appropriate card reader. MMC is the same format that
many digital cameras use. See SD.
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.
Time To Go (TTG): Time To Go is the estimated time required to reach the destination
waypoint. TTG is calculated using the SOG and DTG. See SOG and DTG.
