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Sub-millimetre setting out and surveying by Baseline Surveys Ltd

Thursday 3 November 2022

In this article I’m going to describe how to achieve better than one millimetre of accuracy in setting out and surveying for large or small civil engineering projects, writes Paudie Barry.

After qualifying in 1989, l went to work in London to gain professional experience. I was hired as an assistant residentengineer on a shaft sinking and tunnelling contract for Thames Water on the London Water Ring main project. What I learnt about accurate surveying and setting out on that project would stay with me for the rest of my life.

Ex-tunnelling engineer

The London Water Ring Main was the largest tunnelling project in the world at the time. My role was surveying the surface and subsurface control station network and site benchmarks in order to check the contractor’s setting out station coordinates and to confirm that the contractor’s calculated tunnel boring machine position at the tunnel face was the same as my coordinates.

This is where I learnt all about high precision surveying and setting out, specialist knowledge, which I’m going to share with you in the following short few paragraphs.

Cool and calm state of mind

The most important element of achieving sub millimetre setting out accuracy is being fully relaxed and focused. It is important to reduce the amount of environmental distractions as much as possible and to fully ignore everything except what you are doing. Turn off your phone and actively discourage any chatting around you unless it’s directly related to the high precision setting out job at hand.

The importance of a robust baseline

Because everything is positioned from the Baseline, it is important for both high precision setting out and surveying, that the baseline should have one set up point and one reference point.

The Baseline accuracy is critical, but even more important is the length of the Baseline. For example if your Baseline is five times longer from the station point than the setting out point is from the instrument, then whatever inaccuracy is in the baseline will be reduced by a factor of five when setting out the point.

This is why length is the primary consideration when setting up Baselines with GPS. GPS is only accurate to about 7mm whereas the accuracy of a total station is at best within 1mm over a 200m distance. This is why using one coordinate and one bearing from that point is a best way of setting up an accurate and an unambiguous Baseline for downstream total station setting out use.

Site survey to determine physical site extent v proposed extent

A precise topo survey of the physical site boundaries and critical features can be surveyed on site while establishing the site Baseline.

Based on this site survey a setting out drawing is prepared in CAD using real world ITM coordinates, which allows us to overlay land registry boundary data and confirm if there is no clash between the proposed development, the site features and the sites neighbour’s land registry mapping data prior to marking out your site.

Setting out from the Baseline

With high precision setting out, care and attention along every single step of the process is the only way to achieve sub millimetre setting out accuracy. Total stations, tribracks and barrels should be freshly calibrated and always handled with great care.

The total station, along with the targets, should be set up accurately over each point and this should be checked by rotating the instrument horizontally and viewing the optical plummet crosshairs and confirming that the cross hairs are constantly centred when rotating the instrument through 360 degrees.

Avoid setting up on tarmac as this may melt and the tripod tip will penetrate the tarmac during the set up.

Setting out using modern survey equipment

As you may already know, Autolock automatically centres the total station cross hairs to the exact centre of the target. This is a very desirable function as it eliminates sighting errors.

A point is selected from the logger screen for setting out and the instrument turns the angle towards that point automatically.

Once the target is 'autolocked' by the instrument, the logger will start the millimetre countdown to the setting out point. Making sure the target is plumb is critical for high accuracy.

No mini prism or 360 degree prism should be used

It’s important to note that a mini prism should not be used for accurate setting out, because depending on the targets tilt or horizontal orientation it can easily be 3mm out, even when plumb. It’s best to use a traverse target and a mini tripod system known as the Marksman.

Transferring the point from the survey target to the concrete

It is a challenge to hold the target on a mini pole plumb while reading the countdown offsets to the point on the data logger. The setting out engineer is required to simultaneously watch the level bubble and read numbers on the logger.

Once the position is confirmed, the target is plumb and in the right place, then the point of the target must be lifted and the point is marked on the concrete. This is a lot trickier than it sounds and it is usually at this stage that most of the millimetres of accuracy are lost.

The Marksman

The Marksman is a targeting system which eliminates the risk of error at this final and crucial setting out stage. Essentially it’s a mini tripod system, which can be levelled and moved along a flat surface to coincide with the exact setting out point.

Once in position, a laser precisely marks the concrete where the setting out point is with an accuracy of less than 1mm, which allows you to hammer a nail or mark the concrete with pencil accurately positioning the setting out point.

About Baseline Survey Ltd

Paudie Barry, company owner is a qualified civil engineer, has a higher diploma in GIS and 33 years of experience as a land surveyor and setting out engineer.