Furuno FSV-30 Operator's Manual page 62

2. HORIZONTAL MODE
Note 1: There must be at least three degrees difference between the starting
and ending points to get a fan-shaped alarm zone as shown in (a) and
(b) below. Otherwise, the unit paints a 360-degree alarm zone as in (c)
where there is less than 3 degrees between the starting and ending
points and (d) where the bearing marker has been rotated one full turn.
Note 2: The echo strength which triggers the alarm can be set with ALARM
LEVEL on the ALARM & AUDIO menu. The setting range is 0-30.
This value corresponds with the number of 32-echo colors.
2-32
How to set the fish alarm zone
More than 3˚
(a) (b)
How to create desired alarm zone
<<ALARM ZONE SETTING>>
PLACE CURSOR AT STARTING POINT
AND PRESS MENU KEY TO SET.
PRESS R/B KEY TO CANCEL.
<<ALARM ZONE SETTING>>
PLACE CURSOR AT ENDING POINT
AND PRESS MENU KEY TO
COMPLETE ALARM ZONE SETTING.
Within 3˚
(c) (d)