DEVICES/INSTRUMENTS USED IN SETTING OUT A BUILDING ON A CONSTRUCTION SITE

Setting out of a building involves marking of a building position, size and shape in terrain to establish all the pegs, lines and levels needed for construction purposes. It can vary from the measurements of angles and distances by a theodolite to the most sophisticated total station and gps. For all this control points are needed .This essay explains how theodolite, gps and total station are used in setting out a building on a construction site.

First of all the first task is to establish a base line from which the whole building can be set out. Baseline are located by setting out their terminals. If coordinated the would be set out from a traseverse. The length and width of the baseline can be measured and compared with its compound value .Altenative baseline could be set out with reference to other details such as road center lines, existing buildings etc. once the baseline has been checked , individual building can be located from it .
Theodolite uses movable telescope to measure angles in both horizontal and vertical planes they are manual instruments that comes in two types , transit which rotates in full circle in the vertical plane and non-transit, rotation in half-circle.Using a theodolite at point A and sighting to the furthest end of the baseline point B ,turn off a right angle and establish a peg at C .locate a peg at E in similar fashion from B .Check the distance CE . with a theodolite at C and using CE as a baseline locate corner D .measure the diagonal DE to check the setting out so far .Adjustments can be made to D if the check discovers an error. Locate corner F from E in similar fashion and check the diagonal CF .check the length DF

Secondly a global positioning system is also used in setting out .if the point is in asuitable place for GPS observation and the transformation between the local co-ordinates system has been established ,then real time kinematics GPS can be used to steer the operator to the desired point ,usually to an accuracy of acentimeter or two prolonged observation at that provision will then determine its position to a higher accuracy and an appropriate small movement can be made to improve the location of the point .when points have been set out and a proper monument of its position established it is often prudent to carry out further checks to ensure that the mark is in correct place.It can be checked by means of differential GPS .prolonged observation from the point after it has been established will give a more accurate result than that obtained by the real time kinematic method .However to guard against the possibility of systematic error in GPS observation or data processing it may be desirable to check the result by some completely independent means i.e the measure of angles and distance with respect to nearby known stations.Typical builders or sites square can be used and the Pythagoras theorem for calculation.setting out using GPS is agreat way of pegging out a boundary line over a large open area of ground it can be used to set building corners for site clearance and footing

Lastly a total station is a modern surveying instrument that integrates an electronic theodolite with an electronic meter. It reads slope distance from the instrument to any particular spot and also it measure angles.Total station must be levelled and then centered over a central point in the same way as for a theodolite as before this must be done correctly in order to for the instrument to give correct results . Having set up the total station it has to be oriented horizontally to the site coordinates system and it may also have to be oriented vertically .an adjancent control point is the chosen as a reference point . Inorder to describe the position of a point polar coordinates are required these need a line and an angle .The total station measures polar coordinates these are calculated as Cartesian coordinates within a given orthogonal system either within the system its self or subsequently in the office the distances can be checked by use of Pythagoras theorem
An angle is represented by the difference between two directions ,the horizontal angle between two directions leading to the points A and B is independent of the height difference of those two points provided that the telescope always moves in astrictly vertical plane when tilted ,whatever its horizontal orientation .The vertical angle is the difference between the direction of zenith and the direction to the point under consideration .The vertical angle is therefore correct only if the zero reading of the vertical circle lies exactly in the zenith direction .Therefore total station can be set up on the existing station and sighted onto another control station to provide a reference bearing .The angle to be turned through and the distance to be measured are calculated by simple geometry. For example the instrument is turned through an appropriate angle and a target is moved is moved until it is on the total stations line of sight and at the appropriate distance which places it at or close to a desired point ,many total stations can perform these calculation automatically given the coordinates of the two controlled stations and the new points.

The most common ultimate purpose of a set out points is to act as a predetermined position on the ground or partly built structure to mark where the foundation points should be built for the new construction hence there is need to make sure that the set out was correct and the building foundation is standing in the required direction this can only be achieved with the use of a theodolite ,gps and total station