Humminbird 561 Operation Manual page 19

Inverse is a method where weak returns are
shown with dark pixels and strong returns with
lighter pixels. This has the benefit of ensuring that
weak signals will be clearly visible on the display.
Structure ID® represents weak returns as light
pixels and strong returns as dark pixels. This has
the benefit of ensuring that strong returns will be
clearly visible on the display.
WhiteLine™ highlights the strongest sonar returns
in white resulting in a distinctive outline. This has
the benefit of clearly defining the bottom on the
display.
Black (Bottom Black) displays all pixels below the
bottom contour as black, regardless of signal
strength. This has the benefit of providing a high
contrast between the bottom and other sonar
returns on the display.
12 What's on the Sonar Display