Trying to understand my sonar?

[james/loch lomond]

For the past season i have been using my sonar (eagle optima)in the fish arch mode i am now trying to understand what is being displayed on the screen. In the course of a days trolling i might see 10 to 12 true arches but i also see lots of "dots" on the screen , i know they are fish but is there any way of working out the size of the fish ie: if the sonar is showing a group of "dots" at 20 ft deep are they the same size of fish, if they show the same thing at 100 ft deep, or does the size of things change in relation to the depth/screen size eg: 240 pixels at 24 ft compared to 240 pixels at 240 ft depth, how does it go?
any help on this will be greatly appreciated..thanks ..james

[Gumbo]

LAST EDITED ON Jun-28-01 AT 10:00AM (CST)[p]The determining factor of the size of the arch displayed is how much of the cone the fish passes through. If a fish swims through the cone, entering on one side, swims through the center, and passes out the opposite side, then you will get a large, perfect arch. However, if a fish passes just through an edge, you'll still get a return signal but not an arch, and not a big one.

It is my understanding that if you have two fish of equal size swim right through the middle of the cone--one at 20' and the other at 40', the arch at 20' should be larger than the deeper fish, though not necessarily twice as large.

So the size of the arch is determined by the amount of fish in the cone, not necessarily the size of the fish. Though some of today's powerful, hi-res displays can display a "belly" on some arches, indicating big fish. Also, fish in a school seen together can also show different size fish.