Splines might be a different matter, as Basemaps come under Objects. The only thing that I might experiment with, based on your experience with splines is to initially move up and back down again on placement, see if that sets the height. Daft thing is, the bug is entirely random as you can lift loads of terrain on the area covered by the Basemap, nothing happens. Go back and do another bit and up it goes.
A long term solution would be hi-res mapping/plan drawing on the terrain itself which would eliminate the need for basemaps entirely, just import the plan directly from bmp or jpg. Maybe in TRS2028...
A long term solution would be hi-res mapping/plan drawing on the terrain itself which would eliminate the need for basemaps entirely, just import the plan directly from bmp or jpg. Maybe in TRS2028...