RESOLVED: Basically, pilot error. Somehow I had 'Apply displacement' enabled for the Image Map Shader with an amplitude of 1.35. It's a mystery to me, but that was the problem. Unchecking the Apply Displacement brought everything together...though I'm still curious why I could not manage to get the grass to sit on the terrain even when linked to the Image Map node.
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I'm working on a scene that has sand covered terrain, with rocks scattered about. Basically the node tree is: compute terrain->Base Colours->(an Image Map)->(sand and rocks merged)->Planet 01
When I bring in a grass population, it hooks to the output of the "Compute Terrain" node, which I suppose is the default. The problem is the grass is beneath the sand/rock/image map terrain...if I tilt the camera to view beneath the surface, there's the grass, about a meter underneath. I've tried setting the grass node to 'sit' on the Image Map node, the Sand node, and Base Colours, as well as the Compute Terrain--but it never moves, just stays underneath.
Can someone suggest what I may be missing? I've looked, but can't spot anything glaringly obvious.
Thanks.
You can hook the grass to the last node before planet, that way it takes all displacements after compute terrain into consideration.