The Datum Line

Before a plan exists there is a line. In a section drawing it is written as ±0.00 — the datum — and every dimension on the sheet is measured from it. Choose it badly and the whole drawing argues with itself.

We pick the datum on site, not in the office. Usually it is the threshold: the height at which a person steps from ground into building. Everything above it is shelter and everything below it is engineering.

The discipline this imposes is useful beyond the drawing. It forces an early decision about how the building meets the ground, which is the decision most often deferred and most expensive to change.

It also produces a quieter architecture. When every level is a whole number of steps from one reference, the building acquires a rhythm nobody has to explain.