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Appendix G
Understanding the Graph
Whiteline
The GPSMAP 178/178C can help you to determine if the bottom is hard or soft. When the sonar
soundwaves are reflected 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 significant objects lying on the bottom. The unit uses the whiteline function to
make this bottom layer information easier to distinguish.
With the GPSMAP 178/178C, 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.
F
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