Simrad AP28 Operator's Manual page 78

The symbol in front of the depth reading will change to
indicate that the depth is measured from:
Press the MENU key to confirm the offset setting.
78 | Setup at installation
the keel
Range
-10 - +10 0 – ±5: 0.1
5 – 10: 0.5
or
Step Default value
line
the water
Units
0.0 m, ft