Glossary
GPS & Navigation Terms
25
Estimated Position Error (EPE):
Estimated Position Error is a calculation that
indicates the potential position inaccuracy the user may experience due to a
variety of GPS factors which include satellite position in the sky, signal strength
and others factors.
Fix Type:
Fix Type indicates whether the GPS receiver is providing 2D fix or 3D
fix. A 2D fix requires only three satellites and provides only latitude and
longitude. A 3D fix requires four or more satellites, and provides latitude,
longitude and altitude.
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
.
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