If space permits, the grid for a simple framed structure would be marked as an offset all around the site with survey monuments or nails in the road. When the base slab is cast this would then be transferred to the main slab as marks at each end of the gridline, as well as intermediate marks for ease of use by tradesmen. These marks are then replicated on each floor see Section 4.
These are then used by all trades. Therefore when deciding on a grid layout, some advance thinking is required to ensure grids are suitable for all trades. With everybody working from the same. Figure 4. This is often laid down in the company quality assurance plan. We can further aim to reduce these errors in the way in which we use instruments by following observation routines. Observation routines, as well as cancelling out many errors present in instruments, can also show their magnitude.
This not only gives ongoing checks that the calibration of the instrument is within acceptable limits, but also helps to give some confidence in the equipment and results obtained in using them. A good way of ensuring that an instrument remains in adjustment is to look after it. Most instruments look fairly robust and many are even waterproof. However, an instrument contains delicate and fragile components capable of achieving a high degree of accuracy.
Remember, a good total station costs more than the annual salary of most junior engineers! If the lid does not close easily, you have got it wrong! Forcing the lid back on a box is likely to damage the instrument. An open box fills with water, dirt and dust.
This may be transferred to the instrument when it is returned to the box. Do not wander about with it. Do not put an instrument on a tripod without securing it as it may get knocked off its perch. Carrying an instrument on a tripod will quickly put it out of adjustment and you run the risk of collision damage.
When setting out you need all the help you can get to achieve accuracy. Maltreatment of an instrument will quickly erode tolerances as the instrument will be out of calibration. You may return to find some- one has pulled a power lead, which has ruined your instrument.
It will not be his fault! If you need to leave an instrument, unclip the tribrac and retain the set up. Alternatively, an instrument can be returned to its tripod once removed.
As long as the legs have not been disturbed, all you need to do is level and re-centre the instrument. Better to leave an instrument on the floor rather than risk it being knocked from a desk. These may be wrenched away and in doing so may move the instrument. Many are neither—especially so as they age. It is therefore advisable when setting out in the rain to try to keep the instrument dry. A wet instrument must be dried before returning it to its box.
A wet instrument left in a box will sweat causing water vapour to damage the instrument. Think how instruments work and treat them sympathetically. Such boxes are expensive to replace.
Should the box contain an instrument, flexing of the lid might damage the instrument. This should be carried out at leisure and only by an experienced person. It is very easy to end up with an instrument further out of adjustment than before you started.
The inexperi- enced should be wary that adjustment screws are very small and easily sheared off. Assembly bolts will need periodic tighten- ing. If these are allowed to loosen too much the tripod will cease to be a stable platform, and will produce observation errors. When constructing the pyra- mids it is said they filled the excavations with water and fat. It is not necessary for us to flood the site these days.
Our fat line can be achieved with an optical level, which gives a level reference plane. The height of the fat line, or height of collimation, is measured. All levels are measured from a datum level. The datum usually used is the ord- nance datum OD which is derived from mean sea level as measured at Newlyn in Cornwall, UK. A local site datum or a GPS derived datum may also be used. Datum values measured to the ordnance datum are noted as above ordnance datum AOD. In mining and tunnelling, in order to avoid negative values, the datum value will have, for example, m added to it.
On site, the site datum will be established as part of the site control. If that task falls to you, it will be necessary to find at least two OS Ordnance Survey datums and level from them to the site and back. It is always advisable to find more than one as a check.
This will confirm you have found the datum to be correct and will identify any relative error between them. Should the error between the chosen datums be large it would be advisable to check against a further datum. If your site is to tie in to another e. Levelling by conventional means is usually carried out using a dumpy type level, tripod and levelling staff.
The level will probably be automatic i. Other levels in common use Digital electronic levels that read a bar-code staff. These operate using the same principle as optical levels, and can be used in either digital or optical mode. When used in the digital mode a bar-code staff is read to 0. Results, as well as being calculated by the level as work pro- ceeds, can be downloaded into a computer. This type of level can be very efficient for taking many readings e.
Downloading into a computer also saves time and reduces input errors. These instruments are not suited to all conditions, particularly when there are obstructions to a clear line of sight. Being an electronic instrument, they also require a battery. Water levels, although very low-tech, can be very useful when working in con- fined spaces or to allow tradesmen to transfer their own levels, e.
They can also be used where other instruments may not be suitable e. Water levels are most suitable for transferring a level to another point, e. Rotating laser levels are often used by tradesmen. A laser level produces a level plane of laser light see Section 4. Levels are then transferred in the same fashion as with optical levels. Once set up and occasionally checked by an engineer they can easily be used by tradesmen without constant intervention by an engineer.
They are of particular use to bricklayers, concretors and those installing. Additionally engineers working alone can make use of such devices. An optical level provides a level line of sight to an accuracy that is related to the qual- ity of the level. A level plane can be achieved by rotating the level. In transferring a level from the site datum the level needs to be set up in a position from where the.
A set-up approximately half way between the datum and the point to be levelled is ideal as this will minimise instrument errors. Knowing the instrument height, the level of other unknown points can be recorded. The staff reading at point A to be surveyed is 1. If this is deducted from the height of the line of sight i. If another point is required further away, knowing the height of point A, the instrument can be moved to set-up 2 and the procedure repeated.
Now that point A is of a known height, the procedure can be repeated to estab- lish the height of point B. This time the reading onto the known point called the backsight is 0.
The staff is now placed on point B and read. This is called the foresight. The foresight reads 1. This procedure can be repeated for as many points as necessary.
To transfer a datum it may be necessary to have several set-ups. When the points levelled are used only to transfer the level over a distance of several set-ups, and are not required in their own right, these are called change points.
For every reading that is taken there will be a small amount of error will be incurred. The more set-ups taken away from the benchmark the more error will creep in. Point A This acts as a check on the accuracy of the levelling, and ensures that no gross errors have been incurred e. An acceptable closing error will depend on the task in hand. A series of readings taken from a single set up, are called intermediate sights.
Each reading is taken in turn from the HPC height of plane collimation. A levelling staff can be used to obtain readings from above and below the HPC height of collimation. For example, to take the level of a ceiling soffit, invert the staff so the zero is on the point to be measured, and read as before. When calculating levels, inverted readings are treated as negative values, i. When booking these readings it is essential to identify that they are an inverted reading so that it is not forgotten when it comes to calculating the result.
Conventional practice is to put a ring around the reading. As the height of collimation is the most com- monly used method and being slightly easier to grasp , this method is covered here.
It is assumed that you have correctly levelled the instrument on firm ground and that the instrument has not moved during reading. Most of the errors come from the staffman, so it is essential to ensure the staffman is correctly briefed and a firm eye is kept on what he is doing.
Always ensure the staff is plumb see Fig. A staff rocked gently through the vertical will give a true reading when the staff is showing its lowest value as this will be when the staff is plumb. Electronic instruments are just an electronic version of older now scarce mechanical equip- ment. Although each one is different in appearance and software, they are all basic- ally very similar see Fig.
Some manufacturers ensure that software is similar, not only between models of instrument in the range, but also similar to compatible systems such as GPS.
A theodolite measures both horizontal and vertical angles. A total station measures horizontal and vertical distances as well as angles. Total stations usually incorporate useful software and most can receive data from a computer.
Once set up they are capable of calculating their position from known points and, in addition, can calculate bearings and distances for points to be set out. Some instruments have motor-driven directional telescopes that can if required lock onto the prism pole and take a reading. For example, this could be employed on a survey of a wall into which your building has to fit.
After the start point and stop point have been entered, the instrument will then survey at your specified inter- vals and record all readings. Readings can then be downloaded into a CAD pack- age for further plotting work.
Technology advances at a startling rate. Whilst this relieves the hard-pressed engineer, it is important to understand the basic principles of changing techniques. A blind acceptance of the capabilities of a new instrument or piece of software is an invitation for trouble. Inevitably there will be a time when for some reason it gives you results that are not correct. Huge errors are easy to spot. The nearly right results are the hardest to find, and those most likely to go unnoticed until its too late.
When using a total station for the first time, do not be daunted by its high-tech capability. Remember it is just a theodolite which measures distances, and it has a built-in calculator.
Until used to it and all its functions, re work calculations long- hand as a check. This will help you to understand the way in which the equipment is meant to work. Without experience it would be possible, for example, to mistake the order of the Eastings and Northings on the screen.
Setting out with an instrument on the ground is similar to setting out a technical drawing. To commence you need a reference line from which to work. The coordin- ates of the points you wish to set out can then be calculated relative to this reference line. The angle and distance on the page would be measured with a protractor and. EDM operates by sending an electronic beam from a telescope to a reflector or reflective surface, which then returns the beam back to the instrument so that it can calculate the distance to the centre of the prism pole or surface concerned.
The display can be changed to show a variety of information i. Other makes are similar. In this instance it is a laser optical plumment which shines a laser onto the floor, prism and target plate. How accurate is it? Most, but not all, instruments display distances to the nearest millimetre. That indicates that distances will be accurate to 3 mm plus another 3 mm for every m or part of of displacement from the instrument.
This accuracy can be rapidly degraded if good practice is not followed. For example, if a prism pole is held out of plumb, then tolerances will be quickly eroded. If an EDM signal is too close to an obstruction the signal can be disrupted giving poor results. EDM is also. Current values need to be input to ensure the instrument applies the right corrections to observed distances: the longer the line the more adverse the effects become.
Prisms are not necessarily interchangeable: some prisms have a different prism constant. Using a substitute prism may give incorrect readings causing a constant but systematic error. If in doubt carry out a check over a known distance. The instruction manual will identify how a particular instrument works. Many instruments also have a calibration routine, which allows you to follow a set proced- ure that identifies and corrects for instrument errors.
The more basic instruments will need to be calibrated by using traditional calibration tests. Older mechani- cal instruments can sometimes be adjusted quite simply once the calibration error has been identified.
Instruments should not be randomly adjusted and such adjust- ments should only be attempted by qualified personnel. Instruments are precise pieces of equipment, in the hands of an inexperienced operator they will give inaccurate results. Calibration screws are fragile and may be easily damaged, so should be handled with care. To minimise instrument errors, when reading angles always read on both faces of the instrument.
Observe your backsight your reference target or base line end and record the angle, e. Then, observe the foresight the target to be observed or unknown point and record the angle, e. The amount the instrument differs from face left to face right for the two readings indicates the size of the face error in this case about 6—8 seconds. Some of this error might be due to poor setting up. For accurate work several rounds angles read on both faces of angles need to be taken.
An instrument resolves the slope distance observed to a horizontal dis- tance using the vertical angle. If a large face error is present, this can have an effect on observed distances.
You should be still over the mark, if not, the distance off the point is twice the error. Half this error can then be taken out by repositioning the instrument over the point halfway towards the point over which you are centering.
The bubble should be in the same place, if it is not, the displacement represents twice the bubble error. Re-level the bubble eliminating out half the error. It is important to choose the correct equipment for the task and to be aware of its limitations.
Although EDM is an excellent technique, it is not suitable for measuring very short distances: these are better measured using a tape.
Likewise a tape is not the best equipment for measuring m across a littered construction site. A pocket tape will probably be no better than class II. The class is usually identified near the end of the tape by a roman II ringed by an oval. Do not use anything less than a class II tape for setting out see Fig. For accurate measurement, use a class I tape. As with the pocket tape, this is identified by a roman I inside an oval near the start of the tape. Class I tapes have a blank end section so the zero mark can be lined up with the point to be measured.
This prevents errors generated by not picking the correct starting point for tapes, which are gradu- ated from the end. Ensure that your chainman or assistant correctly holds the end of a tape to remove the possibility of error. Class II tapes are identified in the same manner but have their zero at the end of the tape rather than a blank section. Some engineers prefer to work from the 1 m mark to avoid end errors. If in doubt, double check.
Unclassified tapes should not be used for accurate work. Fibreglass and fabric tapes should really only be used for very approximate work. Steel bands, although not commonly used, are capable of high degrees of accuracy. All these factors may affect on the length of a tape. Should temperatures in use vary considerably form the calibration temperature then suitable corrections will need to be made.
The length of a steel tape will vary by 0. The calibrated tension is marked next to the class of the tape and is applied with a spring balance. You can correct for this by meas- uring the angle of the slope and calculating the horizontal distance. Alternatively you can measure the slope in a series of short horizontal steps.
Complicated tap- ing exercises should be avoided, if possible, due to the potential for errors. Correct tension should be applied. Short distances should be checked with a tape.
For example in setting out vertical formwork or transferring levels up successive floors of a multi- storey building. There are several methods for dealing with this, which include:. Plumb-bob A string or cord line attached to a heavy weight will, under gravity, hang vertically and provides a quick and reliable method of checking or transferring a plumb line. Such a technique is applicable to plumbing a lift shaft or in other indoor situations. Moderate oscillations of the plumb line can be counteracted by damping the plumb-bob in water or oil.
If oil is used it should not be disposed of into the drainage system. Where the height to be plumbed is considerable, wire may be substituted for cord. Care should be exercised when lowering a plumb line that the safety of those work- ing below is not compromised. It may be prudent to use a small weight to initially establish a line and then replace the weight by a heavy bob for final plumbing. Spirit level This is only suitable for small-scale work, for example in checking door frames or concrete formwork.
Where a spirit level has been used for an approximate check, more sophisticated methods should be used for a final check. Theodolite This instrument can be supplied with a diagonal eyepiece to enable the user to look ver- tically upwards. With such an attachment, the theodolite can be used for checking the alignment of columns and in many other similar applications.
By using a tape in con- junction with the theodolite it is also possible to measure the extent to which a member is out of plumb. The ease with which the telescope of a theodolite can be set to move in the vertical plane makes it a very useful instrument for use in this type of work.
An optical plummet is an instrument which sights directly up or directly down. It houses an automatic compensator similar to an automatic level which gives the instrument greater accuracy than is achieved using a theodolite with a diagonal eyepiece. Designed for the purpose, the optical plummet will achieve excellent results see Fig. On some sites the site grid will be tied into the Ordnance Survey OS national grid.
This may be due to it being a road, railway or some other large project that needs controlling on a large scale both before and after the construction phase of the project. The grid on the site may be little different from map grid references used when reading an ordinary OS map. More often than not though, the site grid will be a local grid designed to be on the same orientation as the main column layout or some other major feature of the project.
This is done for ease of use for dimen- sioning the works. It is useful to note that jobs designed using a CAD system, are able to provide coordinates at the click of the button.
On jobs where, for example, you have piles to set out all referenced from some obscure point on a skew grid, it is worth making a formal request for the information in coordinate form.
Armed with coordinates, a site control network with coordinated values and an instrument. Mark perspex plate with felt tip pen. Principles The zero of the coordinate system is always on the bottom left of the map.
It is customary to quote the Easting before the Northing. However American practice is to quote Northings first. In practice, avoid hav- ing grid origins so close to any work to preclude negative values.
Figures should be to three decimal places indicating the desirable level of accuracy for most tasks. This is because to calculate angles may lead to errors, e. This is not entirely true. Lasers have been available for use for a long time. A laser is basically just a visible line of sight, a string line of light. A laser is classed by its power output and may be class 1, 2 or 3.
Class 1 lasers Invisible to the eye, needs a laser detector and is the safest class of laser. As Class 1 lasers need a laser detector or laser eye, these are generally restricted to laser level type of use. The laser is then used in the same fashion as a level, the laser eye can be attached to a boning-rod or staff. This can be given to the operative who is controlling the excavation, levelling a line of wall copings or other opera- tion. They can also be used in machine control systems e.
The engineer will however still need to set up the instrument and calculate rod heights etc. Class 2 lasers Visible line of light. The beam must not be observed through an instrument, or stared at. Safety precautions: terminate beam at the end of its useful length not a rule but advisable ; avoid where possible setting up at eye-level.
Class 2 lasers emit a visible line of sight, which make them very useful for a var- iety of tasks. These also come in the form of laser levels, but some can also be used to form vertical and inclined planes. This can be useful for setting wall lines, or ramps. A staff or stick used in conjunction with a laser can be marked with a vari- ety of points to indicate, for example, the bottom of a dig followed by the top of the bedding and then the top of the paving.
The advantage over the Class 1 laser is that a laser detector is not required. It is also possible to get a laser eyepiece attachment for some instruments. This can be used to transform a visible line of site for taking measurements and setting out. An example of this would be to set an instrument on the centre-line of a tunnel. A laser can then be shone through the instrument to give the tunnelling gang a line to follow. With a string line, this would require regular resetting.
Class 2 lasers are also used in pipe lasers for setting out drainage. The laser is set up in a section of pipe at a manhole and the line and grade set for the drainage gang to follow. Class 3a lasers A visible line of light but of higher output than Class 2 lasers. These should not be used where a lower class of laser will suffice due the increased risk to the eyes. Safety precautions are as Class 2 lasers, except that the termination of the beam at the end of its useful length is a requirement.
Class 3a lasers have the same function as Class 2 lasers but are more powerful. In some instances such as on long lines of sight, in poor dust laden atmosphere or where a screen to detect the laser is used , more power may be necessary. There are higher classes of laser for example Class 3b but these are too power- ful for normal use on site due to the risk of eye damage. Consequently the strict safety measures that need to be in place hamper their use.
Consider, for example, a simple rectangular domestic house. The first priority will be to set out the foundations. It may be sufficiently accurate to set out basic excavation dimensions by taping from exist- ing work.
However, it is preferable to carry out a dry run using an accurate method of setting out. This will flag-up any unexpected calculation errors or problems in obtain- ing lines of site. This can be done using coordinates or grids depending on the type of site control in use. If unacceptable errors are found then re-work the original setting out until sufficient accuracy is established. These lines should be close to the building lines so that tradesmen can measure short distances for their work.
Failure to do this may result in foundation malfunction due to an eccentricity of the load. A theodolite may facilitate this work. It is, therefore, essential that the initial setting out is to an appropriate standard.
When setting out rows of houses it is essential to work from the whole to the part, see Section 4. Where engineers have worked from one end it has been reported that the last house in a row could not be accommodated on the remaining plot! That is not true today, with modern technology, systems and software are being continually updated. Nowhere is this more obvious than with satellite posi- tioning. It has exploded onto the construction market changing some operations. As technology improves, accuracy increases and costs come down it becomes more economical to employ it on smaller and smaller jobs.
What is it? Satellite positioning is the determination of the position of a point using a satel- lite receiver. Satellite positioning is generally known as GPS or global position- ing system after the American military system, which was first available for public use.
Unlike most surveying and setting out tasks, the skill required of the operator is minimal. The satellite receiver does all the work in gathering the data and output- ting or storing it as required. With setting out it can provide the operator with pre- determined setting out coordinates.
Accuracy depends on the methods employed and the equipment used. For con- struction setting out centimetre level accuracy is achievable.
This makes it suitable for many setting out tasks. Unlike traditional survey methods, each point is inde- pendent of the points around it, and therefore each point is of a similar accuracy. Degradation of accuracy due to creep with distance from the main station is no longer a problem.
If used in unsuitable conditions, accuracy may be compromised. An error in one point is not passed on to adjacent points.
What are the advantages? When used for setting out, a single engineer with a setting out pole equipped with a satellite receiver can set out points almost as fast as he can mark them. With a road centre-line for example, the operator can walk the route and mark centre- line points at whatever frequency is required. The setting out information can be taken straight from the design on disk without the need to input a mass of figures. Work is unaffected by weather or daylight or a lack of it.
Visibility between points is not required, so local obstructions shrubbery, mechanical plant, low buildings, walls etc. Productivity increases are considerable. As well as giving plan coordinates Eastings and Northings , it will automatically pro- vide heights as a matter of course.
Satellite systems can also be integrated into computer-controlled plant, in which, for example, a grader has the road design in its memory. The grader blade is automatically adjusted to give the correct earth- work profile. This eliminates the need for a complete setting-out team along with their instruments, forest of timber-work, chainmen and their transport. What are the disadvantages?
Cost is always an issue, but this has to be balanced against productivity. GPS is not suited to all locations. Due to the fact the position of the receiver is derived from observing a number of satellites, a clear view of the sky is necessary.
This may make GPS unsuitable for city centre sites shielded by adjacent tall buildings. A received signal may give inaccurate results if deflected off the side of a building. GPS is not suitable for tunnelling work. However GPS can be used very efficiently to establish a control either side of an obstruction under which tunnelling is required.
GPS does not work well in tree-covered areas, again due to the need for a clear line of sight to the sky. The height element of the output is of a lower order of accuracy than the plan coordinates. Additionally, heights given are not above mean sea level as with trad- itional levelling , but above the mathematical model of the Earth, WGS84 World Geodetic System Unfortunately, for Europe this does not run parallel to mean sea level. However the GPS output can be configured to give correct information.
GPS is not sufficiently accurate enough to obtain the 1 mm precision that can be achieved with a theodolite. If necessary check known benchmarks using GPS. Note: This Standard is in several parts and deals with such topics such as measuring stations, targets and checklists for the procurement of surveys and measurement services.
Methods for Determining Accuracy in Use. Note: This Standard is in several parts and deals with such topics as theory, measuring tapes, optical including plumbing instruments, theodolites, lasers, procedures for setting out using a theodolite and steel tape, and the use of electronic distance measuring equipment up to a range of m.
BS Guide to Accuracy in Building. BS Tolerances for Buildings. Note: This Standard is produced in several parts and deals with such topics as basic principles, recom- mendations for statistical basis for predicting fit between components having a normal distribution of sizes, and recommendations for selecting target size and predicting fit. Other related texts Setting Out on Site: D. ICE design and practice guides: The management of setting out in construction: J.
Smith ed. Surveying and Setting Out: F. Bell, Aldershot, Avebury, The aim is to provide information for people concerned mostly with job specification and site management, and to help avoid some common problems that may be identified during and after the construction of flexible roads and other paved areas.
Asphalts and macadams used in the construction and maintenance of flexible roads and other paved areas consist of a mixture of aggregate and binder. The mix- tures to the required specification are usually produced in batches in a manufactur- ing plant.
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Roofing Handbook. Water Chemical characteristics. WasteWater Treatment Disposal. Activated Sludge Systems. Wastewater Microbiology. Irrigation Engineering. Crop Water Requirements.
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