Glossary
GPS & Navigation Terms
130
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.
Route:
A Route is series of waypoints linked together in a specific sequence to
define a navigation path between two points, and saved in the fishfinder’s
memory. Routes are often used when frequently traveling between two or more
locations because it is a faster and more accurate method than repeatedly
selecting individual waypoints each time one travels through the same area.
Routes consist of a Start Waypoint, an End Waypoint, and a series of waypoints
in between. Users can begin navigation at the beginning, the end (to run the
route in reverse) or at any waypoint in between. See
Waypoint
.
SD:
SD is an acronym for Secure Digital. An SD card is virtually identical in
function to an MMC; however, the speed at which information can be accessed
is generally faster, the storage capacity is generally larger, and the card has
security features which can be used to prevent extracting the data from the card.
SD is the same format that many digital cameras use. See
MMC
.
Selective Availability:
Selective Availability (SA) is a GPS system “feature” that
can be implemented by the U.S. Department of Defense (DoD) to degrade the
accuracy of GPS positioning information. With SA active, the GPS is accurate to
within 100 meters 95% of the time. With SA inactive, the GPS is accurate to
within 45 meters 95% of the time. Currently, SA is inactive and the consumer can
enjoy the greater precision GPS provides; however, the DoD reserves the right to
activate SA at any time.
