Humminbird 100SX Single Beam Operation Manual page 23

Sonar targets which are not physically attached to the bottom
may take one of many shapes. Surface clutter is the layer of
water near the surface which is rich in algae and other
growth, and often is aerated by wind or wave action. This
area of water interferes with sonar transmission and often
appears on-screen as regular clusters of individual dots near
the "0" line.
Thermoclines are sharp differences in water temperature.
These are easily identified by the continuous nature of the
return.
When a sonar signal is reflected off the bottom back to the
transducer, there is often enough energy left in the signal to
be reflected off the surface of the water back to the bottom a
second time. Second returns appear as a slightly weaker
bottom representation exactly twice the depth of the primary
bottom return. The second return is most likely to
occur in shallow water and in areas of relatively hard bottom.
operation.
The 100SX uses advanced signal processing to further
evaluate any sonar return between the surface and the bottom.
For instance, a small school of bait fish usually appears as a
dense cluster of individual dots. If the return meets certain
additional criteria, a fish symbol will be assigned. There are
three different size fish symbols used to indicate the intensity of
the sonar return. While signal intensity is a good indicator of
relative fish size, different species of fish have different sonar
characteristics, so it is not always possible to distinguish fish
size between varying species. The signal intensity is
"normalized" for depth so that a small fish close to the boat
does not appear as a large fish symbol.
ID+ adds an additional piece of information to help the user
locate fish. On selected fish symbols, the digital depth of the
designated fish is drawn to help locate the fish vertically. This is
especially helpful in setting bait depths or for downrigger