I have a 4'x20' solar panel on my 27ft AG pool. We built a south-facing rack to mount the panel on. I can either run all, none or part of the flow through those panels. The plumbing is basically some flexible hoses, a few pvc ball valves and some pvc. I will take a picture and post it in a while to show you my set up. The solar panel, combined with the solar cover, make a huge difference in the temperature of my water.

Whether rooftop installation or on the ground makes any difference, I don't know for sure but I wouldn't think so. I think most important is how it will best fit on your property.