As the sounder pings it gets back the sound over a period of time. So at a time corresponding to 30m, it gets no signal at 38m it gets a weak signal and at 40m it gets a strong signal. The sounder then assigns red for high, and greens for lower then puts it on the screen. The next sounder pinging it adds the next column so you build up your picture that you're used to seeing on the sounder.

The A-scope is just the latest of the columns on your sounder, so its what the sounders seeing at this very instant. Instead of having a single pixel to the very right of your display, you can make this latest column a lot larger. What it does, is as well as displaying the colour, it also displays the width. Strong returns are wider and redder, while a weak return is blue/green and smaller. Imagine if you cut the A-scope thing in half (down the line), you'd be looking at a graph of signal intensity over time (which equals distance).

In the above shot, you can see three blips corresponding to the fish school, then the gap, then the hardish bottom, then the 'thickness' of the bottom (well, really just how dense it is). Not that on the right of the standard sounder output its not clear that there is another fish school right under the boat, right now. You have to wait a few seconds to see clearly, while the A-scope is instant. Whether its useful depends on the conditions, angler, spot, etc, but thats what it does.

Summed up nicely. Might have more off a look at that one, especially on drift or anchored up.

Neels