Line, level & plumb — the three references

Every set-out lives or dies by three references. Step through each one on a real building set-out, meet the device that checks it, then compare all three side by side.

Plan view — the string is set to nails on the profiles, true to the boundary. Press play/pause with the motion toggle above.

LINE

⟷ horizontal alignment

Is it in the right place, running dead straight?

What it is

A string line pulled taut between nails on the profiles. It marks exactly where a building line runs — set to a true measurement from the boundary (or an existing building), taken from the drawings.

Why it matters in setting out

Every wall, footing, pad and pier is positioned from these lines. If the first line is out of position or out of straight, the whole building is out with it.

Check it with

Taut string line to nails on profilesTape measure off the boundary

Elevation — different stake lengths, one level datum. The bubble settling to centre and the sight lines show the check.

LEVEL

▭ the horizontal plane

Profiles level across and between one another — one datum for the whole site.

What it is

Level means truly horizontal. On a set-out, the crossbars of every profile are brought to the same height — even on a site with fall — so heights and depths read true from anywhere on site.

Why it matters in setting out

Footings, pads, strips and slabs are all built to heights taken off the profiles. Level profiles give one reliable datum; out-of-level profiles give a different answer at every corner.

Check it with

Spirit level (short spans)Dumpy / automatic level + staffLaser level

Know the limits: a spirit level is only trustworthy over the length of its straightedge — optical and laser instruments carry level accurately right across the site.

Elevation — the bob swings, then settles dead vertical: the corner point up on the strings lands exactly on the peg below.

PLUMB

↕ truly vertical

Transfer a point straight down — from string height to the ground.

What it is

Plumb is the vertical reference — dead in line with gravity. Where two string lines cross at a corner, that point sits up at profile height; a plumb bob transfers it straight down to a peg or into the footing excavation below.

Why it matters in setting out

The lines live up on the profiles, but the building gets built at ground level. Plumb is how the set-out gets down to where the digging and pouring happen — without shifting the point sideways.

Check it with

Plumb bobSpirit level held on an uprightLaser plummet

LINE

Horizontal alignment

Positions and directions of the building lines — set true to the boundary from the drawings.

String line + nailsTape measure

LEVEL

Horizontal plane

One height datum across the whole site — profiles level across and between one another.

Spirit levelDumpy / automaticLaser level

PLUMB

Vertical

Transfers corner points from string height straight down to pegs and footings.

Plumb bobLaser plummet

Quick check

Name the reference — your first pick counts.

1. A dumpy or automatic level on a tripod, reading a staff at each profile, checks which reference?

2. A taut string set to nails on the profiles, measured a true distance off the boundary, establishes which reference?

3. A bob hung from the string intersection drops the corner point onto the peg below — which reference?