7.2.1 Understanding The Echogram Display - VDO MAP 7w Plus GPS User Manual

Variable Depth Marker (VDM)
Horizontal line on to the Echogram window with a depth label. The up/down
cursor keys can move it up and down. The label displays the depth of the
cursor position. It can be moved to any location pinpointing the depth of a
target.
Zoom Bar
Bar located on the left side of the Depth Ruler, showing the portion of the
Echogram currently represented in the zoomed window (on the left part of
the screen). It is turned On selecting the Echo Sounder Split page.
A-Scope
A real time representation of fish and bottom features passing through the
beam of the transducer. It is drawn as horizontal lines whose length and hue
is proportional to the sonar strength returned. When the default palette is
selected, the strongest sonar returns will be shown as the color displayed of
the top of Color Bar while the weakest as the bottom color.
Operating Frequency
Readout of the selected operating frequency.

7.2.1 Understanding the Echogram display

The main elements that can be easily distinguished into an Echogram are:
Fishes
Fishes are represented as arcs because of the cone angle of the transducer.
In fact as the boat passes over the fish the leading edge of the cone strikes
the fish, causing a display pixel to be turned on. As the boat passes over the
fish, the distance to the fish decreases turning each pixel on at a shallower
depth on the display. When the boat is directly over the fish, the first half of
the arch is formed and since the fish is closer to the boat, the signal is stronger
and the arch is thicker. As the boat moves away from the fish, the distance
increases and the pixels appear at progressively deeper depths forming the
remaining half of the arch.
Thermoclines
Are the zones where two layers of different water temperatures meet. The
greater the temperature differential, the thicker the thermocline is shown on
the screen. Thermoclines are represented as horizontal stripes of noise. They
are very important for fishing since often many species of game fish like to
suspend in, just above, or just below the thermoclines.
User Manual
Fig. 7.2.1 - The Fish Finder Echogram
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