I need to create a mesh that is equivalent to the bottom of the entire city mesh. For example, if the entire city was on a completely flat terrain, this mesh would be all of the sidewalks, streets, and lots flattened, and then the normals reversed. so the mesh would not be completely flat because the sidewalks are a bit above the streets and lots. Basically it is a bottom surface for the city that will be used to precisely set the terrain height in Unity.
Any suggestions on where I should start digging?