Construction & QS Civil & Structural Field practice Follow a procedure

Measuring what is already built before you design against it

Surveying an existing structure so the design assumptions match reality, and the dimensions most often wrong.

The engineering question

The design is only as good as the survey behind it. What must be measured, and how do you avoid designing to an assumption?

Worked example

The figures below were produced by running the Trench Excavation & Bedding Calculator with the same inputs .

These figures are worked by the published method rather than executed at build time, because this tool is a self-contained page calculator without a programmatic engine.

A measured survey produces the as-built dimensions that replace the assumed ones on the drawing.
Those dimensions are then fed straight into the quantity calculations, so the takeoff reflects the structure that exists.
The excavation geometry is a useful check: it should agree with the drawings, and a large discrepancy means a setting-out error.

Sanity check

Surveyed and drawn trench depths usually differ by a small amount. A large discrepancy is a finding in itself and should be reported before designing against it.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

RICS — measured survey specification
BS 8656 — building survey