I seem to have generally resolved the overlay/underlay clipping bug. Though the two meshes seemed identical, actually one point was a few pixels off, which apparently makes a big deal. I still would like some general info about overlay/underlay, if you could provide some; i.e. I'm still getting some unusual clipping, i.e. when I zoom out the 'level' of the overlay (the transparent texture on top of the underlay ground structure) seems to rise, but when I zoom into the ground the overlay seems to be lowering with the camera.