Setting out errors can be very costly. A typical example was a swimming pool that could not be certified for international events because its length was out of tolerance.
More common are site surveying errors that cause elements that don't quite fit together, resulting in delays and expensive remedial work.
If a calibrated instrument had been used, the swimming pool would have been built within tolerance. For general survey work, calibration or verification would not entirely solve the setting-out errors on a construction site, but it would help by ruling out one possible (and common) source of error.
The Geomatics Faculty has published revised guidance for calibration and verification of electromagnetic distance measuring (EDM) instruments which puts calibration and verification on a formal footing.
So what’s the difference between calibration and verification?
Calibration is the determination of the systematic errors of an instrument, such that either the instrument can be adjusted to reduce uncertainty of measurements or a correction formula or table that can be used to apply an adjustment to each reading.
Verification is the checking of any instrument against a calibrated instrument, on the same baseline pillars, to see if it performs within specified tolerances. The guidance envisages that relatively few instruments will be calibrated, but all will be verified.
You would be forgiven for thinking that instrument checking was engrained into the surveyor’s psyche.
So why do we need guidance?
The problem is that, without the guidance, a surveyor can send his instruments to a service centre and obtain a certificate, with no legal standing, implying that the instrument is performing within the manufacturer’s tolerance – in the laboratory.
The RICS guidance requires calibration of a survey system (i.e. including prism reflectors) to be made in real world conditions, on an accredited field baseline. Verification becomes the responsibility of the surveyor and has to be carried out on a field baseline which has been established using a calibrated instrument.
Having set the standard, the next challenge is to get it accepted – which is where civil engineers become involved.
Chartered Land Surveyors will be obliged to follow the guidance, but others will not. The challenge is to make it a standard for everyone. The most effective way is to impose a responsibility on all contractors by specifying that they follow RICS published guidance within all contract documents. If we can achieve that goal, everyone will be playing on the same, level playing field and the quality of site surveying work will improve.
The guidance is available for you to download free, while non-members can purchase a hard copy through RICS books.
