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491 Echo Sounder Operations Manual 
longitude
Imaginary lines on the Earth's surface running North/South 
and expressed in degrees (0-180) east or west of the Prime 
Meridian (a line running from the North to South Pole, passing 
through Greenwich, England).
meter
A unit of measurement (length) equal to 39.37 inches, slightly 
more than a standard yard.
nautical mile (nm
)
An international unit equal to 6,076.115 feet (1,852 meters) 
used officially in the U.S. since 1959. A nautical mile is about 
800 feet more than a statute mile. One nm is equal to 1  
minute of latitude.  
ping
A pulse transmitted from the 491 echo sounder. Also, the act 
of transmitting a pulse from the echo sounder ("pinging"). 
SOG
S
peed Over Ground. The actual speed of your vessel relative to 
the earth, not the water.  
tide
The vertical rise and fall of water caused by the gravitational 
pull of the moon and sun. 
transducer
The device mounted through the hull to send and receive 
ultrasonic beams that determine seabed conditions and locate 
fish.
waypoint
A particular location, saved by marking fish on the echo 
sounder’s LIVE-DATA screen, which is used as an intermediate 
or final destination.