Sub Millimetre Setting Out & Surveying by Baseline Surveys Ltd

Introduction

After qualifying in 1989, like most Civil Engineering Graduates before me, I crossed the pond to work in London to gain professional experience. I was hired as an Assistant Resident Engineer on a Shaft Sinking and Tunnelling contract for Thames Water on the London Water Ring Main Project. It was the largest tunnelling project in the world at the time. My role was surveying the control station traverse and site bench marks, to check the contractor’s setting out stations and to confirm the tunnel boring machine position at the tunnel face. This setting out checking role was vital as it would turn out later on.

We were tunnelling a 2.45m Internal Diameter bore (under the Thames) from one shaft in Boorman’s Field to another shaft in Hampton Treatment Works, which was 900m away. The target for our tunnel to hit was a reception chamber with an internal diameter 50mm larger than the external diameter of the tunnel. We had a play of +/-25mm in terms of line and level and as we were 40m below sea level, there was nothing to guide us except our surveying.
This is where I learned all about high precision surveying and setting out and this learning still holds very true today.

 

State of Mind

There are a number of critical components to being able to produce extremely accurate setting out or surveying. The most important component is being fully relaxed and focused, because of this it’s 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.

 

The Importance of a Robust Baseline

A robust Baseline is important for high precision setting out and surveying, the baseline should have one set up point and one reference point. The Baseline accuracy iscritical, but even more important is the length of the Baseline. For example if your Baseline is 5 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 5 when setting out the point. This is why length is the primary consideration when setting up Baselines with GPS. GPS is accurate to about 15mm, the accuracy of a total station is at best within 1mm over a 200m distance.

 

Set up Both Instrument and Target on Baseline

Total stations, tribracks and barrels should be freshly calibrated and always handled with great care. The instrument should be set up accurately over the point and this can 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. When setting up its important to drive the tripod legs as deep as possible to achieve high stability.

Your instrument’s accuracy also has a play, remember every second of arc represents 1mm over 200m. So a five second instrument has an angular accuracy of 5mm over 200m, as opposed to a 1 second instrument being accurate to 1mm over 200m.

Reference points are as important as set up points for high accuracy and a tripod, with a tribrack, rotating barrel and traverse target should be only be used. Again the set up should be double checked and the barrel rotated to confirm the cross hairs are consistently over the centre of the point.

 

No Resecting, no 360 degree targets or using Plumbing Poles For Backsighting

I am strongly in favour of setting up over the set up point and referencing a target on a tripod set up over the backsight point. Using a plumbing pole or resecting between the two points is asking for trouble and should never be done if high accuracy is required. Also only use traverse targets for this operation as the standard 360 degree target can be up to 3mm out depending on orientation. This target is for speed, not accuracy.

 

Modern Survey Equipment

Once set up accurately over the set up point and aiming at the backsight target ,most modern total stations have a function where the telescope can autolock to the centre of the target. This is a desirable function as it eliminates sighting errors, so use it if your instrument has that important function. When measuring the backsight your instrument will compare the measured distance with the coordinated distance between the set up and the back sight points, it is important to note this error/discrepancy as it will determine if you can proceed to the next step or not.

Thankfully, with modern data loggers we no longer need to calculate polar co ordinates from rectangular coordinates, eliminating another source of error. Usually 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 point. Its important not to use a mini prism for accurate setting out, because depending on the target vertical or horizontal orientation it can easily be 3mm out. Its best to use traverse style targets and to carefully orient them towards the instrument.

 

Transferring the Point from the Target to the Concrete

It is a challenge to hold the target on a mini pole plumb while reading the offsets to the point on the data logger. The setting out engineer is required to watch the level bubble and read numbers on the logger at the same time. Once the setting out engineer has 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 millimetres of accuracy are lost.

The Marksman

The Marksman is a targeting system which eliminates the risk of error at this final and crucial 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 point with an accuracy of under 1mm, which allows you to hammer an nail or mark the concrete with pencil faithfully representing 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.
My 33 years of experience and advanced equipment allows me to set out points to an extremely high level of accuracy.

 

Please feel free to contact me with any setting out query you may have.
Paudie Barry 086 2535285
[email protected]