NorthStar 490 Echosounder Operation Manual page 25

Chapter 2 - Quick Start: Using the Echo Sounder
When you select bottom lock/center zoom, the sea bottom is
Using bottom lock/
flattened out and displayed in the center of the screen. You
center zoom
can see any fish near the bottom, as with the bottom lock
zoom, and also examine the seabed's composition.
When you select marker zoom, you can choose the part of the
Using marker zoom
water column to magnify. The top edge of the magnified area
is determined by the location of the depth marker. To move
490 Echo Sounder Operations Manual Rev. B2 Page 2-11