Digital computers have generated additional interest in planning-by-layers. The data structures that they use lend themselves to representing buildings and places as layered structures. In AutoCad, a popular computer-aided design program, the Layer Control command enables the different entities of a drawing to be grouped into layers. A single-storey building might use separate layers for foundations, floor surfacing, services, ceiling and roof. Multi-storey buildings will use a great many more layers. Additional layers can be drawn for other components of the building: heating, ventilation, electrical cables, structural frame and so forth. In ArcInfo, one of the most popular GIS programs, map layers are known as coverages, each of which is stored as a subdirectory. If coverages have the same registration marks (TICS) it is easy to perform overlay, sieving and buffering operations. The LIST command, which is available in both ArcInfo and Autocad, will show a database listing of coordinates for every node. LIST shows them in database format. From ArcInfo, it is possible to transfer the data into a standard office database or spreadsheet program, such as dBase, Excel or Lotus.
Until recently, all architects and planners made their proposals on two-dimensional maps. Some still do, but the practice is unlikely to survive. It came with the Renaissance, and computers have made it obsolete. The change is of profound operational and conceptual significance for architects, planners and landscape designers. Environmentalism and structuralism make planning-by-layers conceptually appealing. Computers make it easy. If a CAD program is used, the layers are likely to be geometrical. If a GIS program is used, the layers will also be thematic. When the boundaries between the two software technologies begin to dissolve, there will be a very wide choice of what to show on layers. With three powerful reasons for adopting a layered approach, there can be little doubt that the procedure has a long way to go.