Tuesday, June 28, 2011

The accuracy fallacy

“I hate accuracy in CAD!” I remember a friend declaring 2 decades ago. I also recall, early in my career, small-scale-builders giving me a hard time on construction sites for dimensions on my drawings ending with 2,3,7 (mm);
Accuracy is a tricky thing – a double edged sward. It is not an interchangeable term with dimensioning, however the two are closely related.

Over the recent times, often guided by PI insurers, design consultants have developed an almost pathological fear of dimensioning. You can leaf through hundreds of sheets of IFC sets with very little dimensioning on them, to then find yourself on ‘setout’ drawings that have random dimensions all-over the place, including positions of section-markers and loose furniture.

“Traditionally” a dimension meant an instruction: somebody thoughtfully set out grids, openings, fittings in all 3 directions. There also existed a common knowledge of relationships and min dimensions for typical details.

In CAD/BIM world, dimensioning became an after-thought – if the CAD file was accurate, a somewhat automated afterthought.
This on its own should’ve not been a problem if the thinking had gone-in when the digital file was originally prepared, i.e. walls were set out to some logic, same for openings.
Tolerances taken into account between various elements and a 3D grid set in stone.

Applying uncontrolled dimensions is a dangerous practice, BIM/CAD accuracy will not help you there.