Okay, new day new ideas.

markschum - I used a separate ground prop because I want the dolly camera at (0,0,0) and my ground slightly below that - the default Poser GROUND prop has no way to adjust its height. I made the default Poser GROUND prop invisible (Visible, Visible In Raytracing, and Cast Shadows are UNchecked; Visible In Camera is checked) so it shouldn't be having any effect... but I guess it could be some odd interaction. Definitely worth trying a couple of tests.

But before that...

I recall that large values for 'Scale' can sometimes cause odd things to happen, so I tried reducing my groundplane's scale from 1250% to 100%. The bottom parts of the N, E, S and W renders are no longer clipped, but the right half of the groundplane in the W render is still not showing. Here's the W, N, E, S renders butted together (renders were 500x500 - I got the same result with a 2048x2048 render, so I don't think render dimensions have anything to do with the problem

I'm wondering whether my groundplane having only 4 vertices (1 face) may have something to do with it, so it's probably worth trying with a mesh groundplane - the Poser GROUND prop for example.

(sidenote: the reason I'm using a 4 vertex ground plane is that it enables me to use displacement to make the ground bumpy. If I use a mesh I get artefacts along the mesh grid if I'm using AO - example image of that effect here http://www.renderosity.com/mod/gallery/index.php?image_id=2557344 )

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.