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Activities of Engineering Surveying for Proposed Land Reclamation Project …

- - - - Calculation of whole circle bearings
        
        The first part of a traverse calculation is to check that the observed angles sum to their required value. The observed angles of polygon traverse can be either the internal or external angles and angular misclosures are found by comparing the sum of the observed angles with one of the following theoretical values.
        
        sum of internal angles = (2n-4) x 90
        or sum of external angles = (2n+4) x 90
        where n is the number of angles measured.
        
        when the bearings in a link traverse are calculated, an initial back bearing can usually be determined from known points at the start of the traverse and to check the observed angles a final forward bearing is computed from known points at the end of the traverse. The angular misclosure in a link traverse is found using
        
        sum of angles=(final forward bearing-initial back bearing)
        x (m x 180)
        
        for polygon traverse when working in an anticlockwise direction around the traverse, the left-hand angles will be the internal angles of the traverse and when working in a clockwise direction, the left-hand angles will be the external angles.
        
        forward bearing = back bearing + left hand angle
        
        the whole calculation of whole-circle bearings must start with a known bearing or an assumed arbitrary bearing.
      - Calculation of coordinate differences
        
        The next stage in traverse computation is the determination of the coordinate differences of the traverse lines.
        
        The information available at this point will be the whole-circle bearings and horizontal lengths of all the lines.
        
        The fractional misclosure is always computed for a traverse before adjustments to the coordinate differences and the station coordinated calculated. This is to compared to its specification and a decision made to accept or reject the traverse. If these checks are successful, adjusted values are obtained by applying the correction to the calculated values.
        Notes : All methods of adjustment to traverse will alter the original bearings and distances entered on the traverse form by small amounts. If the bearings and distances between the traverse stations are required for any setting out or other work, they must be recalculated either from the adjusted coordinate differences or the adjusted coordinates using polar to rectangular conversions.
      - Calculation of coordinates
        
        to calculate the coordinate of the stations for polygon traverses, the coordinates of the starting point can either be allocated to give positive coordinates for a site or to align it with a structure or they may already be known if an existing station is used to start the traverse.
        
        to calculate the coordinates if the stations for link traverse, the coordinates of the start and end points will be known and the coordinates of the new stations will be relative to these.
        
        For both types of traverse, the coordinates of each station are obtained by adding or subtracting the adjusted coordinate differences as necessary, working around the traverse. This continues until the coordinates of the start point for a polygon are recalculated or the coordinates of the end point in a link traverse are calculated, at this point, the final check is made on the traverse calculation.
        
        For a polygon traverse, the final and start coordinates should be the same.
        
        For a link traverse, the final coordinates should be the same as those of the end point.
- - - - Fieldwork for Stadia Tacheometry
        -set-up instrument over station & record the height of instrument (height of the trunnion axis from the ground)
        
        select one if the adjacent station as the reference object (RO) and if possible set the horizontal angle to zero, all angles will be relative to this RO
        
        when the staff is at a point of detail, observe and record the three stadia hairs, horizontal and vertical angles (nearest 1 minute is sufficient)
        
        the procedure is repeated for all details needed and as a precautionary measure, it is sensible to check the reading to the RO from time to time.
- - - - Reference Grids : enables points to be set up over a large area.
        
        The survey grid is drawn on the survey plan from the original traverse or network. The grid points have known eastings and northings related either to some arbitrary origin or to the National Grid. Control points are represented by the original control stations.
        
        The site grid is used by the designer. It is usually related in some way to the survey grid and should, if possible, actually be the survey grid, the advantage of this is that the original control stations have been permanently marked then the designed points will be on the same coordinate system and setting out is greatly simplified. The grid itself may be marked with wooden pegs set in concrete, the interval between grids being small enough to enable every design point to be set out from at least two and preferably three grid points, but large enough to ensure that movement on site is not restricted.
        
        The structural grid is established around a particular building or structure which contains much detail, such as columns, which cannot be set out with sufficient accuracy from the site grid. The structural grid usually established from the site grid points and uses the same coordinate system.
        
        The secondary grid is established inside the structure from the structural grid when it is no longer possible to use the structural grid to establish internal features of the building.
      - Offset pegs : located on the lines of the sides of the building but offset back from the true corner positions.
        
        Whether used in the form of a baseline or a grid, the horizontal control points are used to establish design points on the proposed structure. For example, the proposed corners of a building have been established by angle and distance methods form a site grid. However, as soon as excavations for the foundations begin, the corner pegs will be lost. To avoid having to re-establish these from the horizontal control points, extra pegs known as offset pegs will be used.
        
        These pegs enable the corners to be re-established at a later date and are often with profile boards in the construction of buildings.
      - Baselines : a line running between two points of known position.
        
        In some cases, the designer may specify a baseline that runs between points on two existing buildings. Design points are then set out from this line either by offsetting at right angles or by measuring distances from points on the line.
        
        The accuracy of the baselines method increases if two baselines at right angles to each other are used. Design points can be established either by measuring and offsetting from both lines or a grid system can be set up to provide additional control points in the area enclosed by the baselines. The use of two baselines in this way leads to the use of reference grids on site.
  - - - In practice, the datum used is defined by establishing a physical point on site known as a master bench mark (MBM). Details of the actual point used will be agreed in writing between the Engineer and the Contractor.. The MBM is either given an arbitrary reduced level, for example 100.000m, or its reduced level relative to Ordnance Datum Newlyn (ODN) is determined. Once the MBM is established, lines of levels can be run from it for two main purposes :
        
        First, to establish points of known reduced level near to and on the elements of the proposed scheme. If these points are based on an arbitrary datum they are known as temporary bench marks. If they are based on ODN they are known as transferred bench marks. Both often referred as TBMs. Although TBMs often located in the new positions of the scheme, any existing horizontal control stations can be used as TBMs providing that they have permanently marked.
        
        Second, if it is intended to use nearby Ordnance Survey Bench Mark (OSBM), their reduced levels are checked with reference to the MBM and, if there is any discrepancy, their amended values are used. This ensure that the overall vertical control remains with the MBM.
- - - - Calculating volumes from spot heights and contours
        Volumes from spot heights
        
        used in 'localised' construction, such as earthwork for the construction of buildings.
        
        example : cutting and filling of earth for basement (car park), underground water tanks to obtain required formation levels.
        
        a grid is formed over the area and the reduced levels of the grid points are determined.
        
        the difference between the grid heights and the proposed heights are obtained in order to estimate volume.
        
        Volumes for contours
        
        Suitable to be used when large volume is involved such as volumes of water contained in a dam, soil, heap.
        
        The area of contour surface is determined. The contour's surface can be treated as a cross-section whose areas can be calculated either graphically or mechanically.