5/8/13 Setting Out

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Setting Out

According to many Insurance Underwriters setting out carries a high risk

which is reflected in premium costs when arranging Professional

Indemnity.

In any form of geo-spatial engineering there are three main categories of

error which are generally described as mistakes, systematic errors and

random errors. At GM Surveys we aim to reduce the risk of errors and

mistakes by operating a strict Quality Control system consisting of a

simple Ten Point Check sequence. Our TPC identifies and eliminates the

different kinds of error that can lead to expensive delays and correction

costs therby minimising the risk to our clients. From the moment we are

engaged to set out any structure or infrastructure we investigate as

many apsects of the design and setting out data as possible to ensure

the project runs smoothly and on time.

Our Ten Point Check (TPC) sequence is shown below. Click on the links to

find out more.

1 Survey Instruments

2 T opographic Survey

3 Architects and Engineers drawings

4 Setting Out Data

5 Setting Out Control

6 Automated Setting Out

7 On Site Computations

8 Gross Error Checks

9 Fine Checks

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10 As Set Out Survey and Report

Survey Instruments

Without accurate and correctly adjusted equipment a Surveyor or Site Engineer cannot perform his tasks to the satisfaction of the client. It is

therefore of the utmost importance that the equipment used on site is adequate for its purpose. All total stations owned by GM Surveys are

calibrated annually by professionally trained technicians and are covered by serv ice maintenance contracts. Calibration certificates are issued at

the time of serv ice and can be forwarded to our clients upon request. Other equipment such as optical plummets, tribrachs and levels are also

serv iced on an annual basis by local dealers. We consider every part of our kit to be equally important - it does not matter how accurate an

expensive total station is if the tribrach that attaches it to the tripod is out of adjustment. It is a simple matter to ensure survey accessories are kept

accurate and in top condition all the time so our staff are trained to check and adjust them if required. Optical levels are checked at least once a

month and before any major levelling exercise using the recognised 'two peg test'. For setting out where low accuracy is required a standard detail

pole is used and the verticality of the pole is checked daily. When a higher degree of accuracy is needed a laser plummet is used on a quick set

tripod. The laser is also checked for accuracy before each days work. Survey Control Units are kept on maintenance contracts to ensure they are up

to date with the latest software from the manufacturers. GPS equipment does not need to be serv iced as there are no moving parts although we do

maintain an annual subscription which allows us to receive GPS data quickly and efficiently using the Trimble VRS now system.

Topographic Survey

Probably the most important drawing to ensure any construction project is located correctly within the boundaries of the site and at the correct

level is the T opographic Survey . Our first v isit to the proposed development will include a check on the physical boundaries of the site and other

hard points of detail such as building corners as shown on the Survey. The client should have prev iously confirmed that the physical boundaries are

the same as the legal boundaries. This is important as it sometimes happens that a fence or a wall can be erected and assumed by a new landowner

to be a legal boundary when in fact that may not be the case. See the section on Boundary Surveys for more information regarding boundaries.

We also check general ground levels in the area of proposed construction to ensure existing levels agree with design levels. ten minutes doing this

can prevent costly delays if drainage has to be redesigned due to incorrect levels shown on the Survey.

Architects and Engineers Drawings

We realise that great care has been taken by the Architects, Structural and Drainage Engineers in the production of their drawings but it is inev itable

that the odd discrepancy between drawings will get through so there is no harm in an extra pair of eyes checking them. Before we begin processing

our setting out data we will request copies of all drawings in CAD format. The Architects and Engineers site layouts will be overlaid and we will check

for general continuity before rotating them (if necessary) to fit perfectly over the original Survey grid. It is often the case that a critical dimension is

missing from the drawings when we are asked to set out points on site that have not been prev iously processed with the rest of our setting out data.

If the Architect or Engineer are not available at the time it could lead to delays before they can be contacted and we can return to site. However if

we have the CAD drawings loaded onto our laptops and/or tablet PC's we can quickly interrogate the drawings whilst on site to obtain the

additional dimensions we require. We also ensure that the drawings we have used to process the setting out data are the same rev ision as the

drawings on site. If we find the drawings we have are vastly different from the latest rev isions we can request they are e-mailed directly to us on site

from the Architect.

Setting Out Data

We aim to process as much setting out data as possible in the office prior to our first site v isit. It is far easier to concentrate in a quite office than in

the noisy and dusty environment of a busy construction site. Having prev iously overlaid the Architects and Engineers layouts to match the Survey grid

every foundation, wall, kerbline and drainage point is now directly related to the position of the control stations. Using Trimble T erramodel software

we assign each point to be set out a unique name and co-ordinate in either 2D or 3D as required then store them in entity specific files. When all the

data has been processed it is transferred to the fie ld control unit by USB, data stick or e-mail. This eliminates the possibilty of human error that can

occur when typing co-ordinates manually and the time saving can be substantial if the files contain hundreds, or even thousands, of points. Finally we

produce setting out drawings for each phase of the construction which can be left on site with the contractors as a permanent record of the

points we have set out.

Setting Out Control

The old 'grid' system which has been used for many years and is still favoured by a few companies is outdated and inefficient. Many sites are squeezed

onto the smallest plots of land where there is barely enough room to work, let alone store materials and ensure the safety of the setting out control. A

site engineer will spend hours carefully placing grid pegs and profiiles around the site knowing they will almost certainly be disturbed or obstructed

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at some stage of the construction. The use of theodolites and tape measures can also be counter productve due the amount of set-ups required to

complete a relatively simple task on a busy site with materials stacked on every spare metre. Accurate control is essential for any setting out

project. A network of control stations and reference objects will be established using the original Topographic Survey control as a base. Where

possible we will relate these stations to OS National Grid using GPS equipment so that in the event of a total loss of control we can quickly replace

them without the loss of accuracy. RO's will be placed around the site on solid, immovable objects (i.e. walls, buildings etc.). The combination of

control stations and RO's enable us to establish temporary 'v irtual' stations for a days work in almost any position on site. If a contractor decides our

total station is in the way it only takes minutes to move it and quickly establish a new v irtual station. Our control units have powerful on board

software which immediately indicate any errors every time the instrument is set up. If the errors are within a pre-determined tolerance the software

will make adjustments to the instrument readings to ensure that a high degree of accuracy is maintained.

Automated Setting Out

Having prev iously processed the setting out data there is no need, in most cases, for a Surveyor to spend hours sitting in a site office with a pile of

drawings and a calculator. Our Surveyors will already have the necessary information loaded onto the field control units and will be seting out within

minutes of arriv ing on site. Once the total station has been positioned and checks made to the surrounding control points the control unit will, quite

literally, speak to the Surveyor and tell him which direction to move until he is in the precise location of the point to be set out. Robotic instruments

mean we can use one man teams for some projects although when a large volume of setting out is needed in a short time we recommend a two

man team to speed things up even more. By pre-processing the co-ordinates of each point there are very few manual calculations to be made on

site and taping errors are non-existent as all angular and distance measurements are carried out automatically by the total station.

On Site Computations

Despite all our efforts to pre-process the setting out data it is inev itable that someone on site will ask us to set out something that we have not co-

ordinated. A laptop or tablet pc holds the current drawings for the site while the control unit is in itself a computer with software that can handle

almost any trigonometric calculation. If we get to the site and we find we have out of date drawings we simply phone the Architect and ask them to

e-mail the latest drawing direct to the laptop. Alternatively someone in our office can e-mail an up to date co-ordinate file direct to the laptop or

even to the control unit. However since our aim is to reduce the risk of error from human error we ensure that every point computed on site is

immediately checked as described in the 'fine checks' section and checked against the original setting out drawing as described in the 'as set out

survey' section.

Gross Error Checks

Gross error checks are simple taped dimensions that are compared to written or scaled dimensions on the Architects layout. On our first v isit to the

site we will set out crucial elements of the proposed structure and check dimensions to the site boundaries or other permanent points of detail. These

checks will immediately tell us if the building is in the correct position, the right way round and orientated correctly to the survey grid. They are

recorded on a printout of the layout and stored on file.

Fine Checks

Once the days setting out is complete the distances between points are checked by tape to prove they have been set out correctly in relation to

each other. These can be random checks or a complete record of distances between every point. They are compared to the relevant Architects

drawing and can be written on a paper copy of the setting out drawing for storage on file. This check has the added bonus of prov ing the instruments

used in the setting out are in adjustment as any mis-alignment of tribrachs and optical or laser plummets would be revealed if the distances between

points proved different to the design distances.

As Set Out Survey and Report.

The final check of our TPC system is the 'as set out' survey. By usimg a total station linked to a dedicated control unit the position of each point as set

out is stored automatically in an ASO (as set out) file. At the end of each day the ASO file is downloaded onto a laptop or PC at the office,

porocessed and overlaid onto the master setting out drawing. Each point is compared to the original design position and the file is stored to give us

a permanent record of the date and accuracy of every point we set out. Any discrepencies found are reported immediately to the relevant

authority on site and corrected at the earliest possible opportunity. relevant discussions held on site are written down in a report and stored on file

along with the fine checks taken prev iously in section 9.

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