I used different c-trace based placement onto terrain, or terrain+terrain models, or terrain+terrain models+buildings etc. what is post adjusted according to should it snap to tile xy or z or not. It is different for each entity category (4 kinds of foliage, 2 kinds of building, and terrain models).

Now I use Carlos3DGS's contribution to determine mouse pointer's view line intersecting the zero level plane if c-tracing fails. Some adjustment is still needed but works fine.

And thanks, yes, that temporary info text looks badly sometimes, but it is worthy to be kept, I will add a black rectangle or something behind it...