Humminbird 515 - Manual (Page 17)

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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.
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. Any targets such as fish,
structure and thermoclines will be shown using the Structure ID® method.