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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.