NorthStar Echo Sounder 491 Operation Manual page 24

• marker zoom
The original unzoomed echoes are on the right side, and the
magnified area is shown on the left side of the screen.
A tab below the ZOOM key shows which zoom mode is in use.
When you select bottom zoom, the contour of the bottom is
Using bottom zoom
magnified, clearly displaying canyon edges and echoes near
the seabed. The 491 automatically locates the seabed and
displays it on the lower left side of the screen (normal echoes
are displayed on the right side).
If the seabed moves out of the lower half of the screen (for
example, if you travel into deeper water), the 491
automatically repositions the bottom back into the lower half.
You'll see the zoomed image jump up or down from time to
time, as the 491 keeps the seabed displayed on the screen.
When you select bottom lock zoom, the magnified sea bottom
Using bottom lock
is flattened out and displayed on the left side of the screen at
zoom
the lower edge. You can see any fish that are swimming near
the bottom.
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491 Echo Sounder Operations Manual