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Contents

Section 1 Introduction

Section 2 Linear Survey

Section 3 Setting up an Optical Level

Section 4 Booking Levels

Section 5 Producing Contours & Longitudinal Sections 



 

Description: This unit introduces the techniques used on site to collect data in order to produce a plan of a site.

Author: Gates MacBain Associates


Section 1 Introduction

This Module is designed to complement your lectures and the practical work within your course.  It does not replace the lectures or experience that you will need to get by carrying out the practical tasks. You may find it useful to reinforce or clarify the tuition you receive on your course.    

This unit covers the techniques used on site to collect data for: 

  • A two dimensional plan of an area
  • Determining the variations in levels and effectively adding the third dimension

We will also explore in detail the techniques which might be used to lead finally to production of drawings, contours and longitudinal sections of an area.

 

Abbreviations

CAD

Computer aided Design

 

EDM

Electromagnetic Distance Measurement

GPS

Global Positioning System

 

H

Horizontally

OBM

Ordnance Bench Marks

 

OS

Ordnance Survey

TBM

Temporary Bench Mark

 

TS

Total Station

V

Vertically

 

WCB

Whole Circle Bearing

 




Section 2  Linear Survey


Aims and Objectives

At the end of this section you should be able to: 

  • Use measuring equipment to collect and record data
  • Check the accuracy during and after the survey
  • Produce drawings using the collected data

In the previous Unit we reviewed a range of surveying processes and used the example shown below (Figure 1) in relation to linear or two dimensional surveying. You will note that the plot of land shown by the dotted line contained a driveway, house and garage. It was divided up using a grid of triangles (shown in red) with each corner of the triangle given an identification letter. For the purposes of explaining the data collection we will identify the central triangle and A, B and C as shown below and note that Station D occurs along line CA.       

 Fig_1_LSL

Figure 1: Survey Points.    


The layout of the triangles as shown above is sketched in the survey book showing accurately the relationship between the survey lines, house, drive and garage. Ranging rods are placed at each survey station and a traverse along each line is carried out to measure the length of each survey line and then record features using either offsets at right angles to the survey line or by using tie measurements. Triangles should contain angles of not less than 30º. 

Using the equipment described in Unit 1 the linear survey along line AB is carried out and data record in a survey book usually with each page relating to one survey line. The survey data for line AB would therefore be typically as shown in Figure 2. Note that we use an (/) rather than a decimal point when recording data on site.  

 

 

Figure 2: Data Collection Line AB  

The process is repeated from Station B to Station C with an accurate straight line measurement between the two ranging rods and the Abney Level is used to indicate the slope between the two points.  

We will assume that the slope distance BC is measured as 30.920m and that the slope is measured as -2º from B to C. Typical data collected from line BC is shown in Figure 3.   

 

Figure 3: Data Collection Line BC 

 

The tri-lateration is completed for this part of the survey by traversing between Station C back to Station A. You will note from the layout of the triangles that Station D is located on line CA. Station D is to be used in collecting data from the southern and south east corner of the survey. 

When traversing from Station C back to station A the measured slope distance was 69.100m and the Abney Level showed a very small slope of +0.5º from C to A. Typical data collected from this survey line is shown in Figure 4.  

 

 

 Figure 4


You might find it a useful exercise to produce a drawing using this data. A scale of 1:250 will fit an A4 sheet. Before you begin drawing remember to adjust the length of each main survey line to take account of the slope. Using line AB as an example, the procedure used is as follows:- 

Data available : Slope length = 66.230m  and  Slope =  +1.5º 

True Length =  Slope Length   x  cosine of slope (degrees) 

True Length =  66.230   x    cos 1.5º             = 66.207 

Adjustment on line AB =  66.230   minus  66.207     =  23mm          

Fig_4A_LSL

 

















Conventional Signs  

The way that items are shown on a map is by the use of symbols, examples of which are shown below. You can see more of these in Surveying for Construction listed below.     

 

 

 Figure 5: Mapping Symbols 


Constant Errors 

The length of the tape used on main survey lines should always be checked to ensure that it is giving a true reading. If a tape length is inaccurate then a constant error is automatically factored into the survey. 

If for example a 30m tape was used in the above exercise and this was found to have stretched by 25mm then the slope distances must be adjusted before converting slope distance to true distance. For example line AB at 66.230m with a constant error of +25mm per 30m will in reality be:-  

 66.230m   minus   ((66.230m / 30m) times 25mm)    

=   66.230 minus 55mm                                                                             =   66.175m (Adjusted slope length)   

In this case the true length (taking account of the constant error) would be:-  

66.175   x   cos 1.5    =   66.152m     

Using the above procedure the adjusted values on line AB are now as shown with the true length in brackets.

  


Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapters 2 & 3)


Self-Assessment Task

  • As part of a group allocated by your tutor you are to conduct a small linear survey in a small area on the Campus using the above techniques and then produce a drawing at an appropriate scale.




 

Section 3  Setting up an Optical Level


Aims and Objectives

At the end of this section you should be able to: 

  • Competently set up an optical level ready for use and accurately read the staff.

Most modern tripods are made of alloy and may have traps tied around the legs for easy transportation. Setting up the tripod involves standing it upright, releasing the screws clamps on the legs and extending the top of the tripod to about the forehead level of the user. The screw clamps are tightened; the legs are spread and pushed firmly into the ground. Selected clamps may now be loosened to allow rough levelling of the top of the tripod and these are of course re-tightened prior to fixing the instrument. 

Attach the level to the tripod using a brass screw thread and handle which is part of the tripod assembly. The screw housing on the base of the instrument ensures that the instrument is securely fixed. You can see how an optical level is setup by clicking on the Video link below: Setting up an optical Level.     

 

 

 Figure 6: Optical Level   

 

Key points in levelling the instrument read for use: 

  • Level the instrument by adjusting the levelling screws.
  • The bubble (not visible on the above image) should be central in the circle.
  • Turn the telescope through 180º
  • The level is accurate if the bubble remains in the centre of the circle.
  • If the level is inaccurate the bubble will move outside the circle
  • Parallax is the apparent movement of the cross-hairs over the levelling staff when the eye is moved up and down while sighting through the instrument.
  • To eliminate parallax the eye piece should be perfectly focused on the cross-hairs and the
  • Telescope should be perfectly focused on the levelling staff.

Reading the staff  

  • Sight onto the levelling staff.
  • Focus the telescope on the staff.
  • Always ensure the bubble is central
  • Take the reading of height.
  • Each square is 1cm (10mm)
  • First metre is black and second metre is red
  • Each E is 100mm (0.1m) apart

 

Figure 7: Levelling Staff

 


Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapter 4)


Self-Assessment Task

  • Set up an Optical Level and take a reading and ask your tutor to check that it is set up correctly and verify that your staff reading is accurate.





Section 4  Booking Levels


Aims and Objectives

At the end of this section you should be able to: 

  • Use optical equipment to accurately record the data, book, reduce and check levels.

There are two methods of booking levels: 

  • Collimation Method: The height of collimation is the height of the horizontal line of sight above the datum, and is sometimes called the height of instrument.
  • Rise and Fall Method

 

Advantages of the two methods of booking 

The advantage of the collimation method is that it is ideal for setting out reduced levels on site. The reduced level of points/pegs on site can easily and quickly be found by measuring down (or up, for inverted reduced levels) from the height of collimation. 

The advantage of the rise and fall method is that it is the simplest method of booking and checking the calculations on site.  

 

 Fig_8_LSL

Figure 8: Diagrams showing booking Procedure  

 

This survey is booked as shown below. It starts and ends on the same OBM   

  

Table 1  


Rules of Booking 

  • Backsight  - the first reading from a new instrument position. The survey starts with a known level. This will be an Ordnance Bench Mark (OBM) or a Temporary Bench Mark (TBM). It is good practice to make the final reading for the complete survey to be at this point to check accuracy.
  • The  Backsight is added to the reduced level to give the height of collimation, entered on the same line.
  • An  Intermediate sight will occur between the Backsight and Foresight
  • The Foresight is always the last reading from an instrument position
  • The foresight or intermediate sight is subtracted from the height of collimation to give the reduced level, entered on the same line as the foresight or intermediate sight.
  • The height of collimation only changes when the instrument is moved to a new position.
  • Every backsight reading gives a new height of collimation, entered on the same line.
  • All readings referring to the same point on the ground are entered on the same line.

The reducing of the levels using the above rules is shown in Table 2    

 

 

Table 2  

 

Rise and Fall   

Rules used when booking levels 

  • The first (and last) reduced level is on an Ordnance Bench Mark (OBM) or a Temporary Bench Mark (TBM)
  • Following line by line down the page calculating the rise or fall between consecutive staff readings.
  • A rise occurs if the first staff reading is greater than the second staff reading in any consecutive pair of staff readings.
  • A fall occurs if the first staff reading is less than the second staff reading in any consecutive pair of staff readings.
  • Add the rise or subtract the fall from the preceding reduced level to obtain the new reduced level, entered on the same line as the rise or fall.

 

 Table 3 Rise & Fall Calculations  

 

Accuracy Issues 

For all types of survey the accuracy of level values should be as follows: 

  • Site TBM relative to Ordnance Survey bench mark ± 10mm
  • Spot levels relative to TBM within 10mm on hard surfaces 90% should be to ± 5mm.

If the closing error exceeds these values the survey should be repeated. 

 

Checks on Calculations 

Check on reduced levels obtained from backsights and foresightsSum of backsights - sum of foresights = first reduced level - last reduced level.  

 

Rise & Fall  

Sum of backsights - sum of foresights = first reduced level - last reduced level = sum of rises - sum of falls = first reduced level - last reduced level.   

 

Checking the Level for Accuracy (Two Peg Test) 

A level must be checked regularly to ensure that it is accurate. If it is inaccurate adjustments must carried out by a specialist. Checks to optical levels must be carried out as follows:

  • Set up the level half way between two pegs 60m apart
  • Ensure that when the level is rotated throughout 360º that the circular bubble remains in the centre of the black circle.
  • Take a staff reading to an accuracy of 1mm on each of the two pegs (A & B)
  • Calculate the difference in level
  • Move the level to a position just beyond B, ensure the level is accurately set up and take staff reading on each peg.
  • Again, calculate the difference in level and this value should match the difference when the instrument was midway between the pegs.
  • If there is a variation of more than 3mm over a 60m length then the optical level needs to be sent back to the maker for repair/calibration.

     


Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapter 4)


Self-Assessment Task

  • Carry Out a Leveling Survey on the Campus which involves at least THREE changes of instrument position. Begin and end on the same TBM to assess your accuracy.




 

Section 5  Producing Contours & Longitudinal Sections


Aims and Objectives

  • At the end of this section you should be able to:
  • Produce contours and longitudinal sections from collected data.

As we saw in a previous Unit we often use a grid of levels to produce data which can be used to: 

  • Produce contours
  • Form longitudinal sections

The diagram below shows a 10m grid of levels on our site.    

 

 

Figure 9: Grid of Levels   

 

The level survey shown has been completed using a 10m grid. The spot levels within the grid are as shown on the grid below. You will notice that four levels are missing as these were within the confines of the house.   

 


Figure 10 :Reduced Levels in the Grid   

 

 

Table 4: Booking Sheet: Levelling Survey  

 

Contours 

A contour is a line shown on a plan which joins equal heights above ordnance datum.To determine the position of these contours it is necessary to use some basic calculations which are often given the grand title of linear interpolation. The grid below (Figure 11) shows the position of the contours within this site and a few examples of interpolated data.     

 

 

 












Figure 11  

 

 

 

Figure 12 

 

We will show the process to determine the position of  the 62m contour with the 10m squares in the grid now given an alpha numeric identification. 

Starting at square A2 we need find where on the west side of the square the value of 62m can be located. It lies somewhere between 62.305 and 61.110. See if you can follow the logic of the following calculation:   



On the north side of A2 which is south side of A1 the values are 62.305 & 61.805

Therefore (62.305 62 / 62.305 61.805) x 10   =  6.1m from the higher value   

On the west side of square B1 the values are 63.790 & 61.805

Therefore ( 63.790 62 / 63.790 61.805)  x 10  = 9.02 from the higher value 

 

On the east side of square C1 the values are 63.200 & 61.395

Therefore ( 63.200 62 / 63.200 61.395)  x 10 =  6.648m from the higher value.  

This process continues until all the contours are in place. 

 

Longitudinal Sections 

Let us assume that we required a longitudinal section through line YY.    

 

 

Figure 13  

 

Longitudinal section through YY using the following data:-  

Distance (m)  0        6        16      22      32      38      48      57

Level (AOD)   64      63      62      61      60      59      60      61    

 

 

 






Figure 14 

 

The data above (Figure 14) is produced by starting at distance 0 and then noting the distances that line YY crosses each contour. The values at the beginning and end of the section were set at 64 and 61 respectively whilst the low point was assumed at 59.5m AOD. 

It is possible by using this process to produced longitudinal sections in any direction once the positions of the contours are established.      

 


Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapters 5 & 6)


Self-Assessment Task

  • Carry out a Levelling Survey in the form of a grid and produce a series of contours. Your tutor will then draw a line across the contours from which you will produce a longitudinal section.






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