Raymarine E90W User Reference Handbook page 171

Bottom Lock is selected for individual fishfinder windows and can
either replace (ON) or appear alongside (SPLIT) the standard
fishfinder image.
Enabling Bottom Lock
From the fishfinder main screen:
1. Use the softkeys to select an appropriate preset.
The softkey name changes to ADJUST...
2. Select the ADJUST... softkey.
3. With dual frequency screens select FREQ1 or FREQ2 to select
which frequency display you wish to change.
4. Select the SELECT VIEW softkey.
5. Select the BOTTOM LOCK option.
Adjusting the bottom lock range/position
With the fishfinder in bottom lock mode:
Using the fishfinder
1. Use the appropriate softkeys and the trackpad to set Full /
Split screen, Bottom lock range and Bottom shift values as
appropriate.
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