Hi guys

I've encountered quite a weird problem and I have no idea what I can do to prevent or fix.

What happens is that when im positioning my polygons in my scene I set them up so that they are intersecting with the ground.
However when I render the scene, in the rendered picture, they are no longer touching the ground, and instead are hovering slightly above it.

Does anyone know of an easy fix for this or of a way to prevent it?

Thanks in advance.