Yeah, what you say is correct Gaeius.

My advice goes for "flat" 2d objects only. Which I got the impression by the picture we where talking about.

By "height" I meant position in Z-axis.

You can group a face and a group, no prob.
If the road is a flat face for ex, there is nothing stopping you from grouping it with the symbols. From there you can glue groups/comps to the face.
Either by dragging premade 2d symbols from comp.browser(with glueing behavior) or using a plugin to reglue..

What I guess here is that you are tracing the road on a face inside a group or on an image entity. This is natural as it looks like. As you trace, SU is creating its own faces with the default material on. As massimo says, it is Z-fighting: the image and the new faces try to display and SU cannot decide.

You can do two things here:

Explode the group/image - and when you are drawing now, it is the face that you are dividing with edges and it will "remain" textured with that image material After you drew your faces, select all (triple click on a face) > sample the projected material from the image (if it is a face with material on it inside a group) with the Alt+Paint tool and apply it on the selected faces. Now everything will look nice 3rd of the 2 solutions: Don't bother yourself with the whole thing until you finish and you will have and easy life. You can always modify those faces later.