How Targets Are Displayed - Raymarine DSM25 Owner's Handbook Manual

14 DSM25 Owner's Handbook
The dark layers indicate a good echo; the lighter areas weaker echoes. This could
mean that the upper layer is soft and therefore allowing sound waves to pass to
the more solid layer below.
It is also possible that the sound waves are making two complete trips – hitting
the bottom, bouncing off the boat, then reflecting off the bottom again. This can
happen if the water is shallow, the bottom is hard, or gain is set high.

How Targets are Displayed

When a target is detected, it is displayed on screen as a mark. The shape and size
of this mark is influenced by a combination of factors:
Boat speed
The shape of the target changes along with your speed. Slower speeds return
flatter, more horizontal marks. As your boat's speed increases, the target will tend
to thicken and arch slightly until at fast speeds the mark resembles a double
vertical line.
The depth of the target
The closer the target to the surface the larger the mark on screen.
Note: The depth of individual targets can be displayed by switching on the Target Depth
ID in the Fishfinder Alarms Setup Menu.
The size of the target
The larger the target, the larger the return on the fishfinder display. The size of a
fish target is however dependent upon the size of the fish's swim bladder rather
than its overall size. This swim bladder will vary in size between different breeds of
fish.
The frequency of the transducer
The same target will appear differently when the transducer frequency is
changed. The lower the frequency the broader the mark.