I ordered a side/down imaging sonar but it hasn't come in yet.  I COULD wait until it is installed and I've had a chance to use it but I'm impatient and the weather stinks.  It seems to me that side imaging would be good to LOCATE suspended fish but the down imaging would help to get better information about their depth, etc.  Am I far off in supposing this?

Also (something else I'll eventually find out) I hear side imaging works best while the boat is moving.  If this is true, why is this?  What does a side image look like when you are not moving at all? 

I like SI for cruising around, and looking for "something interesting," and then use DI to further define that interesting thing.  FOr suspended fish, it helps stay on them, but I prefer regular sonar when fishing over them.

Bottom of pic is sitting still or very slow. Top is moving.

Side is good to show things like this.

Or rocks, brush, and fish

Bluegill beds

I use Di with 2d sonar to compare what I am looking at.

@Wayne P. routinely offers extensive technical information on this matter and may do so here.

On a most basic level, because DI displays what's under you (or what you've passed over)  you'll have to cruise directly over an area to view it in DI.

Conversely, since SI displays what's on each side of your vessel, you are able to get at least an idea of what an area may hold with passing directly over it.

A-Jay