Glass reliefs 
It is possible to simulate a three-dimensional relief via stacked glass layers. Each glass layer, often consisting of a transparent plastic sheet, corresponds to one elevation level. Several layers stacked vertically can represent a three-dimensional object. All layers together form a transparent block and the features depicted seem to be suspended. The steps originating from the discrete layers cannot be eliminated. In such manner, it is possible to easily create reliefs derived from contour maps by gluing maps on the layers: For each contour level, a map cut along the corresponding contour is glued on a new layer. Instead of using maps it is also possible to draw directly onto the glass.
This technique allows depicting even tiny details like the framework of a latticework bridge as well as caves, mines or tunnels.
 This article bases on Terrain models and relief map making. 1956. p.53-62. and Mühle, Helmut. Manuelle Geländemodell-Herstellungsverfahren unter Berücksichtigung des Standes der Technik. 1963. S.44,45,53.