Symbolic representation

Symbolic representation 

I've heard it more than once: "So, what's new about BIM? We've seen it all before, with CAD libraries and the like." People who say such things are not only old, usually in their sixties, but also typically people with little if any hands-on experience about what they're talking. They're not people who spend time drawing, modelling or designing with computers, otherwise they might have noticed a fundamental difference between CAD and BIM: the level of symbols used.

In CAD, the basic primitives one interacts with, the ones that carry the essential information for the representation are graphic elements: lines, shapes, surfaces etc. In that CAD follows established traditions in architectural drawings, which rely on convention to support recognition. For example, two parallel lines close to each other and with a certain length must be a wall. So, in CAD we draw these two lines with vector graphics rather than ink on paper and if we want be sure that there'll be no misinterpretations, we group them together and possibly also label them as a wall. This changes little to the basis of the representation: it's two lines.

In BIM, one may still draw lines but works directly with symbols for architectural entities: one draws a line to indicate the axis of the wall but only to enter certain geometric properties. The type of the wall, its width and layers, are defined by type and not by the other lines. The appearance of the wall may be two parallel lines but that's just one of the many possible ways the wall may be depicted. What the representation knows is that there is a wall, not two or more lines.

This doesn't make BIM better than CAD, it just brings architectural representation closer to the structure of other digital kinds. Texts, for example, have been symbolic at the level of characters from the beginning - not complex pen strokes like in joined-up writing. When I type a letter on the keyboard, what the computer retains is an ASCII code for that character, possibly dressed up with a font, size etc. Just like a wall in BIM, this dressing up determines the appearance of the character; the character is not recognised on the basis of  its appearance.

The difference between BIM and CAD is therefore one of symbol level: BIM uses symbols for architectural entities, while CAD uses general graphic symbols, those belonging to the implementation level rather than the representation proper. There may be many things wrong with BIM but that it finally gives us a symbolic digital representation for architecture is significant.