Background to the problem
Within QGIS I have two shapefiles representing a roads layer and a buildings layer (both taken from OSM).
I now need to create a third 3D shapefile representing the airspace above the roads and buildings. I need to create it as a layer of the above in order that it picks up the same 2D co-ordinates plus height. In other words, I need to create it as a layer along with the two layers already mentioned [This is to satisfy the requirements of GAMA Platform where I shall use the three files].
I need to be able to export the shapefile with a third dimension - it is not enough that I can simply render it in 3D (or 2.5D) within QGIS.
What I envisage is a cube sitting on the above road / building layers. The two current layers (roads + buildings taken from OSM) have a CRS of EPSG:4326 WGS 84. The new cube would cover the entire road network and would have height of 500 meters.
Can you give a really simple step by step guide as to produce and export a 3D cuboid shapefile for use elsewhere?