Microcosm sold over at RDNA has morphing ground planes and a wide variety of textures. They are older and in their Deep Freeze. You may need to tweak the textures a bit for newer version of Poser but the I find many of the textures and the ground planes to be very useful. Terradome and Terradome 2 are also good add ons for creating land inside Poser.

You do have some basic ground planes for Microcosm in the included content. They are limited but will give an idea of how they work.