31 August 2012


Accuracy and precision are often used interchangeably, but they are not quite the same. A point relevant to documenting design intent in Revit (and other software).


The root of accuracy is “to prepare with care”. Exactness is only one possible quality of being careful.  Appropriateness is another possible quality.
When designing and documenting buildings we need to be mindful they exist in the real world, are built by real people, using equipment that interacts with the physical world (as opposed to the virtual).
This starts with the surveyor. Although their CAD file might report to 16 decimal places they haven’t consciously measured to 16 decimal places. You might have noticed their heights and other figures are in metres to 2 decimal places. That is because their degree of accuracy is 10mm; they measure to the nearest 10mm.
On site contractors would like a tolerance of 25mm, but realistically work to the nearest 5 or 10mm. The Australian Building Commission’s Guide to Standards and Tolerances 2007 has a tolerance of 5mm for most things (up to 20mm for a concrete floor).
As an example walls are (usually) made up of a number of elements. Each element has a nominal thickness, and a manufacturing tolerance. In construction the supporting structure is rarely perfectly aligned, and where scrimmed joints occur there is always an extra 2-3mm of thickness. So in reality a wall is never its theoretical thickness (joiners always assume rooms will be narrower when measured corner to corner).
The traditional solution is to dimension ‘zones’ rather than dimensions based on theoretical sizes. A 90 stud wall with 2 layers 13mm plasterboard is theoretically 116mm wide, but it’s zone should be 120mm (note we are only talking about an extra 2mm per side). This ensures room dimensions are measurable by a human, a room with a theoretical width of 4004 becomes 4000 in your documents.
So to document a wall thickness as its theoretical thickness is not being accurate. It may be exact, but it is not “prepared with care”.


Which brings me to Precision. Software doesn’t operate in the real world, it operates in the theoretical world of mathematics. It require exactness to calculate, and in Revit, to maintain relationships (constraints).
Whereas in the real world the exact thickness of a wall is unknowable to within 5mm, in software it has to be known to many decimal places.
If you don’t draw precisely in software it will break. We have already experienced Revit crashing because walls were created 0.00000012 degrees off grid lines. Recently I have seen multiple error messages saying joined elements and other relationships have been broken because a level was changed by a mere 0.125mm.

The root of precision is to “curtail” or “shorten”. To get software to work in a predictable and consistent manner avoid accidental dimensional relationships and very small differences; “curtail” trailing decimal places and unintentionality.

This means thinking about where your building sits on the site, how fits together, how different parts relate.
 It means not blindly tracing data from elsewhere, whether a survey plan, CAD file or rhino import.
A simple rule; leave complexity where it matters (which includes the design), simplify everything else.

So don’t get accuracy and precision mixed up.
Dimensioning walls to their theoretical thickness may be precise, but it is not accurate. You may consider setting your building out from an existing wall or boundary to be accurate, but if that wall or boundary is 0.3 degrees off perpendicular, it is not precise.
When setting out a Revit project, think both about accuracy; how useful the information being created is to others, and precision; how robust is my Revit model going to be.

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