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Appendix G
Understanding the Graph
Whiteline
The GPSMAP 178C can help you to determine if the bottom is hard or soft. When the sonar
soundwaves are refl ected back by the bottom, a hard bottom returns a stronger signal than a soft
bottom. A thin whiteline indicates a softer bottom while a thick whiteline indicates a harder bottom.
Normally, a red/dark line is used to show the point where water meets the bottom. This line follows
the bottom contour, along with any signifi cant objects lying on the bottom. The unit uses the whiteline
function to make this bottom layer information easier to distinguish.
With the GPSMAP 178C, active whiteline helps accentuate where strong signals are located, which
make bottom type determination easier. The example to the right shows the bottom return with and
without the whiteline activated.
)
Whiteline can also help you to determine the type of bottom
structure that is displayed on the chart. By determining the
hardness of the structure, you can make a better informed
decision on the type of structure.
Structure is hard—
probably a rock or stump
Structure is soft—
probably a mud pile
Example 4.
Whiteline Turned OffHard Bottom
Whiteline
Soft Bottom
GPSMAP178CManual.indd 114 3/19/2004, 3:11:14 PM