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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
