Drawing the datum

Before anything on a site can be measured, something has to be declared level — on the cover study, 900mm below the plinth top. The datum is a fiction agreed between a surveyor and a drawing, and every dimension on every sheet afterwards is relative to it. Choosing where to put it is the first design decision on any project, and it is usually made by whoever opens the file.

Put it at the existing ground and the building reads as sitting in the site. Put it at the finished floor and the ground becomes something to be adjusted. Put it at sea level, as the infrastructure engineers will want, and the whole discussion moves into a register where nobody argues about thresholds any more.

Nobody writes down which of those three they chose, because it does not feel like a choice. It feels like setting up the file. But the choice decides what is easy to see later. Dimension from the finished floor and every cut-and-fill question becomes an addition you have to perform; dimension from existing ground and the floor level is the thing that has to justify itself on every sheet.

This is not a metaphor about drawings. It is a description of where the numbers live. A level is only ever a difference between two things, and the datum decides which of them you have to think about.

A datum also has to survive being handed over. On a project of any size there are at least four of them in play: the architect's, which is usually the finished floor; the engineer's, which is usually the top of the structural slab, a construction depth lower; the surveyor's, which is a national grid level; and the contractor's peg, which is a mark on a wall at the edge of the site and is the only one anybody actually builds to.

Each conversion is a subtraction, and each subtraction is a place to be out by the depth of a screed. We have seen this go wrong twice at the same point: a drainage invert set from the structural datum against a threshold set from the finished floor, discovered when the water went the wrong way across a terrace. Both times the drawings were individually correct. Neither set said which zero it was using, because both thought it was obvious.

The fix is one line of text on every sheet, naming the datum and its relationship to the national grid. Almost nobody prints it. It is the cheapest piece of information in the set and the only one that makes all the others legible.

The journal is named for it because the choice is invisible, consequential, and made early — which is the description of almost every decision we are interested in.

There is a second reason, which is that a datum is an admission. Declaring a line level is declaring that it is not, in fact, level in any absolute sense; the ground falls away, the sea moves, the whole thing is an agreement made in order to get on with the work. A profession that was comfortable saying that out loud about its floor levels might be more comfortable saying it about the rest.