Humminbird 688ci HD DI Operation Manual page 178

Estimated Position Error (EPE)
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
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 (Global Positioning System)
GPS provides accurate position (latitude, longitude,
altitude) information virtually anywhere on the earth through satellite technology and
How GPS Works
personal receivers on the ground. See .
Heading
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
Course Over Ground
often confused with Course Over Ground. See .
Map Datum
Refers to a particular survey of the earth's surface that was referenced when
creating a chart (i.e. WGS84). 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 (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. MMC is the
SD
same format that many digital cameras use. See .
North, Magnetic
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°.
True North
See .
North, True
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. See
Magnetic North
.
Glossary - GPS & Navigation 166