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GPS: GPS is an acronym for Global Positioning System. GPS provides accurate position (latitude,
longitude, altitude) information virtually anywhere on the earth through satellite technology
and personal receivers on the ground. A series of geosynchronous satellites broadcast a unique
signal toward the earth once per second. A GPS receiver, 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.