Poser's standard ground plane is square. Mega scenes often require a view of the horizon, which has no corners. After running up against this problem more than once, I think I've found an easy solution.
The pic above has a disk substituted for the ground plane. (A disk is a cylinder with the Y scale set to zero.) I've cobbled together a simple urban texture and bump map for the disk. You can use something more detailed, or even the default ground texture, depending on your scene.
If I end up re-doing this, I'll probably make the outer edges less distinct than the center. For some reason, I did it the other way around. I'll also try positioning the figures closer to the edge, instead of dead center.
Here's the bump map. Poser 4 will ask if you want to convert it to a .bum format. You do.
See what you come up with!