What if you use intersect faces with model? Rightclick your model's geometry even at different heights and a plane or surface or any other geometry that intersects it and rightclick on any of the selected and run intersect faces... Depending on your context you can intersect Selected faces with: Current context (if you're inside a group/component you can restrict to that context) Rest of the model (even if you're inside a context and the rest of model is out of that context); Selection (you intersect your selected objects with themselves) What you can also do is use sandbox tools (if you can't find it on toolbars you must activate it under sketchup preferences > extensions): Select the faces you want to project on the ground and hit drape tool.