Furuno GP-1650F, GP-1650DF Interpreting the Display, Zero line, Fish school echoes, Bottom echo

2.15 Interpreting the Display

Zero line

The zero line (sometimes referred to as the transmission line) represents the transducer’s position, and moves off the screen when a deep phased range is used.

Zero line

Shift

Figure 2-31Zero line

Fish school echoes

Fish school echoes will generally be plotted between the zero line and the bottom. Usu- ally the fish school/fish echo is weaker than the bottom echo because its reflection prop- erty is much smaller compared to the bot- tom. The size of the fish school can be ascertained from the density of the display.

Bottom echo

Echoes from the bottom are normally the strongest and are displayed in reddish-brown color but the color and width will vary with bottom composition, water depth, frequency, sensitivity, etc.

In a comparatively shallow depth, a high gain setting will cause a second or sometimes a third or a fourth echo to be displayed at the same interval between them below the first echo trace. This is because the echo travels between the bottom and the surface twice or more in shallow depths.

The color of the bottom echo can be used to help determine the density of the bottom ma- terials (soft or hard). The harder the bottom, the wider the trace. If the gain is set to show only a single bottom echo on mud, a rocky bottom will show a second or third bottom return. The range should be chosen so the first and second bottom echoes are displayed when bottom hardness is being determined.

Intensity difference in water depth

Second bottom echo

Size of fish school

Small Large school school

Rock base

Mud and sand

Figure 2-32 Fish school echoes

Figure 2-33 Bottom echoes

2-11