Raymarine C-Series Reference Manual page 143

Chapter 7: Using the Fishfinder
3. To adjust the range of the Bottom Lock image, press B-LOCK RANGE and then use
the rotary control to adjust the range.
4. To re-position the Bottom Lock image to view more bottom details, press BOTTOM
SHIFT and then use the rotary control to adjust the shift between the bottom of the
window (0%) and the middle of the window (50%).
5. Press OK or CANCEL to return to the default display.
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