Setting Out, Construction Survey and As-built Verification
This domain tests the transfer of design geometry to the ground and the proof that it was achieved. Marks are earned by showing independent checks, correct datum control and an understanding of grid-to-ground scale, not by describing instrument buttons.
Setting out principles
Setting out begins from the project control network, not from a convenient wall. Establish primary control tied to the geodetic network, then secondary control and grid lines, then set out the works. Every element should be fixed by at least two independent methods or from two well-conditioned stations, and the result recorded on a setting-out sheet with the design coordinates, the observed values and the difference. Tolerances come from the specification; typical structural tolerances are of the order of 25 mm in plan for columns and cores, with verticality checked by plumb at each lift, but the contract figure governs.
Grid distance versus ground distance
HK80 Grid distances are not ground distances. The projection scale factor differs from unity away from the central meridian, and the elevation reduction is about 16 parts per million per 100 m above the datum. At 150 m above datum a 100 m ground distance is roughly 2.4 mm shorter on the grid, and a surveyor who sets out grid distances as if they were ground distances introduces a systematic error that grows with line length.
As-built verification
As-built surveys compare the constructed work with the approved plans and the design model, and record deviations rather than silently correcting them. Outputs typically include plans and sections showing the as-constructed geometry, levels against datum, deviation schedules with tolerance comparisons, and a statement of the method and accuracy. Findings that exceed tolerance must be reported to the designer for a decision; a surveyor does not decide whether a deviation is acceptable.
Worked example
A 120 m long foundation is set out from two control stations. Station A gives a distance 8 mm longer than the grid value; station B, measured from the opposite end, agrees within 2 mm. The discrepancy is traced to a grid-to-ground scale error on the first line. The check from a second station, using a different geometry, is what exposed the blunder; a single-method setting out would have buried it.
Traps that cost marks
Setting out without an independent check. Using temporary benchmarks not connected to the project datum. Presenting a deviation without the contractual tolerance. Assuming as-built means 'as designed'. Failing to record instrument, date, observer and weather on setting-out sheets.