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Contents


Section 1 Introduction

Section 2 Abbreviations

Section 3 Types of Survey

Section 4 Types of Error

Section 5 National Grid

Section 6 Equipment

Section 7 Optical Equipment and its use
 


Description: This unit provides an introduction to the basic principles of Engineering Surveying.

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.

Engineering surveying involves the use of optical instruments and equipment in collecting data for the preparation of drawings and in setting out structures or features during a construction project. There are three basic observables which are measured or recorded within surveying:-
 
  • Distance
  • Height
  • Angle
 
Data for these may be collected in separate surveys in the same area or all at the same time in a single surveying operation. Drawings of an area are subsequently produced and design work completed. The same optical instruments and equipment used to collect the data are often used in setting out positions of buildings, roads and drainage systems etc as required to complete the contract.




Section 2  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 3  Types of Survey


Aims and Objectives

At the end of this section you should be able to:
  • Describes the differences in the three main variables within surveying processes.


There are a number of ways that a survey can be carried out and you should be aware of these and the variables within the surveying process.
 
 
Linear
 
This is the collection of data to produce a two dimensional plan of an area. It involves the measuring the relative distances between existing features. This is achieved by establishing a network of triangles on the site to be surveyed.  Figure 1 shows a house, garage and drive within a property boundary. The exact relative positions of the features within the site can be measured from the survey or base lines shown in red. The geometry of these lines can be reproduced in the office if the correct procedure is used to measure these on this site.   
  
 
Figure 1
 
 
This process is known as trilateration and is working from the whole to the part. The details of the parts as measured from the base lines are used to construct an accurate drawing of the whole site.
 
A quicker method of achieving the same objective would be to use a total station (TS). This would involve placing the TS in the locations shown which are marked using pegs with a nail hammered in the centre of each peg.
 
With the TS set over peg1 it can measure accurately the distances and bearings to any feature within the field of vision. These bearings and distances are recorded on a memory card. All the measurements which can be taken from peg 1 are not shown to avoid a confusing image. The TS is then positioned over peg 2 and a back sight to peg 1 and one corner of the building already measured from peg 1 will allow further distances and bearings to be recorded from peg 2 as shown and any additional points which can be seen. The process continues through pegs 3 & 4 until the required data is collected. The memory card (which uses co-ordinates) is used to transfer the data into a computer which will download the data into CAD which provides the basic drawing.
  
 

 Figure 2
 
 
 
Levelling
 
This type of survey is used to collect height data for an area. It is usual to relate all heights or levels to mean sea level at Newlyn in Cornwall and Ordnance Bench Marks (OBMs) are transferred across Britain to give reference points to sea level.
It is usual on most sites to transfer a Temporary Bench Mark (TBM) on to site from a nearby OBM and this is the start point for any levelling survey otherwise a temporary assumed datum point is identified, marked as such and thereafter used. 

A method of collecting levelling data for a site is to establish a grid of suitable dimensions and each point given an identity usually alpha- numeric eg A3, B5 etc which can be easily applied to a grid. A typical grid is shown below.
  


 Figure 3
 
 
The levelling survey would be carried out by positioning and setting up an optical level (shown as * below) on the site to take staff readings beginning with the TBM(#) and then on the grid as shown until all grid points have been recorded. In order to collect data on all grid points it would be necessary to move the optical level up to a position where point J1 J5 and the TBM could be seen. The final reading would be on the TBM to give a check on the accuracy of the survey. The setting up procedure and booking methods are covered more fully in Unit2.
 
  



Figure 4
 
An elevation of the above procedure at points B1, C1 & D1 is shown below
  


 
 
Figure 5
 
 
 
Setting up the level can be seen in the video/DVD below.


Video / DVD


Self-Assessment Task

  • Briefly describes the differences in the three main variables within surveying processes.





Section 4  Types of Error



Aims and Objectives

At the end of this section you should:
  • List the types of errors that might occur when measuring during a linear survey.


There are three types of error in engineering surveying:
 
  • Gross errors
  • Constant errors
  • Random errors
 
Gross errors are the result of inexperience, carelessness or lack of concentration by those carrying out the survey.
 
Constant errors built into the surveying process. The error will always be the same for any one tape or for any given set of circumstances.
 
Random errors are due to physical and climatic conditions. These errors are usually small and are normally self compensating.
 
In the surveys described above we try to identify and classify some possible errors.
 
 
Description of Error Classification
Gross Constant Random
Tape length is not true x  
Tape is not pulled at constant tension  x  
Misread on the tape x  
Dimension figures mixed x  
High wind bends the tape x
Tape is twisted or misaligned x  
Slopes angles are misread x  
Optical instrument is not set level x  
Staff reading is not correct x  
Booking values in wrong column x  
Not set properly over a control point x  
Angle read or recorded wrongly x  
Angle booked in the wrong column x  
Sum of angles not correct on traverse x  
 
 
Many gross errors cannot be corrected and will lead to inaccuracy in the final drawing. We look further in units 2 & 3 on how to check data for errors and in some cases apply corrections.



Publications 

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapters 3 & 4)


Self-Assessment Task

  • List the types of error that might occur when measuring during a linear survey.



Section 5  National Grid



Aims and Objectives

At the end of this section you should be able to:
 
  • Explain how polar and rectangular co-ordinates are used on site.


The national grid as shown below enables any position in Britain to be fixed. This is done with global lines called longitude and latitude. Longitude run from north pole to south pole whilst latitude run around the circumference of the earth going north or south of the equator. Greenwich Observatory in London is on 0º Longitude with lines going either west or east and the equator is set at 0º with lines going either north or south.
 
Our starting point for the National Grid in Britain is 2º West of Greenwich which the central meridian for Britainand 49º north of the equator. The intersect point of these two lines is known as True Origin. However for the grid to be used effectively the bottom south west corner must be south and west of mainland Britain. For this reason true origin is moved 100 km north and 400km west to produce a point known as False Origin.
 
The National Grid is now formed as shown below with each square being 100km by 100 km



 
Figure 6
 
You will notice that the main letters are based on ST JOHN and that many of these squares are in sea areas. Each main letter covers 25 squares and in the case of T, O, H and J part of their grid is omitted. In the cases of S and N the 25 squares have a second letter which begins with A at the north west, uses all the letters of the alphabet except I and ends at the south east with letter Z. So if we look at the Isle of Man much of the south of the island is in SC but the northern tip is in NX. This two letter reference system will identify an area 100km by 100km.

Each square can now be divided into 100 squares (ten in each direction) now each 10 km by 10km. A square within this grid is identified by starting at the south west corner and first moving east (this is know as an easting) and then secondly moving north (this is know as an northing)
 
Hence we may move 60km east and 40km north so if we were in square SP this would be SP 64. SP64 could then be divided again into 100 squares now with each now 1km by 1km with the bottom south west corner being 60 moving east and 40 moving north as show below. If we move 7km east and 8km north the grid reference is now SP 6748
  
 
 Figure 7
 
 
This process can continue until we reach the level of accuracy required. Details on the National Grid and the Ordnance Survey can be found at the website below.
 
If we can identify co-ordinates points near our site using an Ordnance Survey map then we can establish several control points around the site which relate to the National Grid. Once these control points are established we can find any point for which co-ordinates are known from one of our control points as shown below.
 
 
  
 Figure 8
 
To find the setting out point we need to move 7.873m east and 6.453m north
Hence we have a triangle formed:-
 
 
 Fig_9
  
Figure 9
 
We form the triangle this way to calculate the clockwise angle from north which is known as a whole circle bearing (WCB). This is then calculated :-
 
                  Tan O  7.873/6.453  = 50 deg 39 mins 38.5secs
 
 
So calculations in the NE quarter will give WCBs of 0 90 whilst SE quarter will be 90 180, SW will be 180 270 and NW quarter will give whole circle bearing between 270 and 360.
 
 


Websites


Publications

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


Self-Assessment Task

  • Using an OS map,  produce a six figure grid reference for your College/University.
  • Identify the nearest square to the south west of this location and produce east and north co-ordinates.




Section 6  Equipment



Aims and Objectives

At the end of this section you should be able to:
 
  • List  the types of equipment used in the surveying process


A wide range of equipment is used in engineering surveying. Using the activities above we will look in detail at the optical and associated equipment which might be required. The Fig 10 below shows the equipment that might be used in each case.
 
The equipment required for the survey of a building including internal measurements is covered in another unit.

When carrying out a linear survey the first stage is to walk over the site and decide upon the position of the survey lines and network of well conditioned triangles. 

Ranging rods are used to mark the corners of each triangle and a sketch is drawn in the survey book showing the survey lines and giving each point a station reference (usually alphabetical).
 
 
 
Equipment Type of Survey
Linear      Levelling Theodolite Total Station
Optical Level x  
Theodolite x  
Total Station x
Tripod x x  
Levelling Staff x  
Prism Target x
Abney Level x  
Ranging Rods x x x x
Steel Tape/Band x x x  
Synthetic Tape x  
Arrows x               x x  
Pocket tape x  
Plumbob x x
Wooden Pegs x
Survey Book x  
Levelling Book x  
Record Sheets x  
Pen or Pencil x x x  
   
 
Figure 10






  
 Figure 11: Suveying Equipment
 
Steel tapes or bands are graduated every millimetre and are either 30m or 50m long. They are calibrated to read true length when at 20º C and pulled at a pressure of 50 Newtons force. These tapes are coated with plastic or enamel for protection an ideally should be dried and wiped with an oily rag before being wound into its case. They are used for precise measurement on survey lines. The steel band is similar but is carried on a four arm open frame with a winder attached.

Pocket tapes are usually up to 5m in length and suitable for small offset measurements and the synthetic tapes usually graduated to the nearest 5mm are used to take offset and tie measurements. Because they are made of PVC or fibreglass these are prone to being stretched. Again this tape should be wiped prior to re-winding intothe case.
 
Ranging rods or poles are usually 2m or 2.5m long and divided into 500mm bands alternating red and white. They are used to identify key points and forming straight lines during a survey.
 
Arrows are small steel pins usually about 300mm high with a coloured tag attached to enhance their visibility. These are used to mark survey or offset points as measurements and data collection proceeds. These may be substituted by pegs. There is a weighted version which is used when step measuring. This is known as a dropping arrow.
 
A plumbob is hung below a theodolite to position the instrument over a station prior to fine adjustment with the vertical scope.
 
A wide selection of equipment is available and examples of these can be seen by visiting a supplier such as the one shown below.
 



Websites


Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapters 4 & 7)


Self-Assessment Task

  • Make a list of all the equipment you would require to carry out a linear survey and then collect data for contouring.





Section 6  Optical Equipment and its use



Aims and Objectives

At the end of this section you should be able to:
  • Contrast between automated and basic types of optical instruments.


An automatic optical level  is essentially a telescope which is used to sight on to a levelling staff. It must be set up so that it is level at all points when swung through 360º. It has controls to adjust focus, slowly traverse/clamp and to increase the definition of the crosshairs (black lines on eyepiece used to read the staff).
 
An electronic theodolite is now most commonly used and is essentially a telescope rotating both vertically and horizontally and has the following main features:

 
Focus adjustment on the lens
Crosshair adjustment on the eyepiece
Slow traverse and clamping (both vertical and horizontal)
Digital display of angles (degrees, minutes and seconds)
Vertical scope to position instrument over a nail in peg
Plumbob to position instrument before fine adjustment
 





Figure 12
 
An abney level is a hand held device which when held against a ranging pole and sighted on to the same height on a second ranging pole as shown below, these can accurately indicate the ground slope.
  

Figure 13: Abney Level
 
 
 
Theodolite
 
Theodolites are used to measure horizontal and vertical angles. Most modern theodolites are digital (rather than analogue which requires the surveyor to estimate the reading on a scale) and accurate to within 20 seconds (60 minutes in a degree and 60 seconds in a minute). More accurate instruments can be obtained for very precise work.
 
The easiest way to think of a theodolite is to consider it as two basic elements:
 
  • Horizontal plate of 360 degrees divided into minutes and seconds
  • Vertical plate of 360 degrees divided into minutes and seconds with the zenith angle (vertical and straight up) being 0º and turning clockwise with 90º and straight down being 180º etc.
 
As the telescope is rotated either horizontally or vertically the H and V readings on the digital display will change. A typical view of the readout might therefore be as shown below:
 
 
H    275   40   20
V      87   15   40
 
 
This would indicate a horizontal bearing of 275 deg  40 mins. and 20 secs and a vertical bearing of 87 deg 15 mins and 40 secs.
 
To measure an angle between two points you would position ranging poles or arrows as shown and then calculate the difference in bearings as shown in Figure 14 below:-
 
  
 

Figure 14

Bearings:
Pole 1 =   234 deg 26 min 00 secs
Pole 2 =   256 deg 16 min 40 secs       
 
Difference and angle between Pole 1 & 2 is  21 deg 50 min 40 secs
 
 
Vertical bearings are used to calculate the true horizontal distance between two points.
 
Let us assume that the height of the theodolite was marked on pole 1 and that the telescope was adjusted until the crosshair was on the marked height. The vertical line of sight would now be parallel to the ground slope. If we assume the readings showing on the theodolite were:-
 
H    234   26   00
V      86   45   20
 
If also the measured distance between instrument and pole 1 is 26.853m then:
 

 
The theodolite survey could be carried out on the above site using the same grid. Horizontal and vertical data would be collected for all grid points along with the TBM, each of the four instrument points plus selected points to position house, garage and drive. The instrument positions would need to be marked with a peg and nail.
 
Position 1 is given a real or assumed co-ordinate value and it is usual to identify the true north bearing to ensure that easting and northing co-ordinates are correctly aligned. The co-ordinates and level at the four instrument positions are control points and these are used to determine the co-ordinates and levels of all other points.
 
These four control point can be checked for accuracy as the sum of the internal angles between them must be 360º (see unit 3) and this can be used to validate the survey.
 
The theodolite and tripod are levelled and set up level over point 1 peg/nail using a plumbob. H & V  readings are taken by sighting on to a ranging pole marked with the height of the instrument at:
 
point 1 to control points 2 & 4
the TBM
all grid points which can be seen
any other features on the site for which co-ordinates are required
 
The slope distance between the instrument and the point being recorded is noted in each case. The booking procedure described in unit 3 is used to record the data using record sheets in the prescribed format.
 
 
The theodolite and tripod are moved to control point 2 (peg and nail), levelled and set up over it using theplumbob. H & V readings are now taken on:
 
control points 1 and 3
all grid points in range which have not yet been recorded.
any other features on the site for which co-ordinates are required
                              
Again, the distance between the instrument and the point being recorded is noted in each case including distances between control points.
 
This process is repeated as the instrument is moved to points 3 & 4 respectively which can close the circuit of control points by sighting back to point 1. This is known as a closed traverse.
 
When using a Total Station the equipment requirements are reduced down to the instrument on its tripodalong with wooden pegs to mark the control points and the prism target. Ranging rods or arrows would be used to mark out the grid.
 
It would be good practice to use pegs at the control points so that their co-ordinates are known for future setting out work. The procedure would be as above for the theodolite survey except that data would be saved on the memory card for download into CAD.
 
   
 


Figure 15

Remember zenith (upright) is always 0º and in this case 90º would be a level line.
 
Hence the vertical reading and the slope distance can be used to calculate true distance which is always used when drawing plans. This data can also be used to determine the difference in height between these two points using a simple calculation as shown below:
 
Height Difference =  SIN  3  14  40  x   26.853 = 1.520m
 
 
Total Stations
 
A total station (TS) is an instrument which records three dimensional data directly on to a hard drive or memory card. It achieves this by using electromagnetic distance measurement (EDM) which is achieved by sending a signal or pulse to a target and data is transmitted back to the total station giving:
 
Distance
Vertical Angle
Horizontal bearing
 
Using the on board computer and software it converts this data into usable information and displays the following for the point being surveyed:
 
True distance
Elavation  (height or level)
Co-ordinates from a given start point (Eastings and Northings)
 
In order to achieve the above results a TS must first be set on a known or assumed start point and will need the following initial data:
 
Identity for the start point
Eastings (actual or assumed)
Northings (actual or assumed)
Ground Elevation (actual or assumed)
Height of  target
Height of  TS above ground
 
A back sight on to another known point will orientate the TS to north
 
As each point is recorded it can be coded or identified to help with CAD
 
If the total station has GPS capability it can determine its position in relation to the national grid using global positioning satellites.
 
The video/DVD below will introduce you to the concept of the total station.


Figure 16: Use of Linear Survey Equipment
 
 
Starting at Station A the data from line AB is collected and recorded (See Unit 2) using the steel tape or bandin a straight line between these points. The distance of the base line AB is recorded in the survey book. Where the distance is greater than the tape length a third ranging rod is required to check the direction of travel and an arrow is used to mark this position. The abney level is used to record the slope between AB to allow true distance corrections to be applied when producing the drawing. Synthetic or pocket tapes are used to measure either offset or tie measurements. Arrows are often used to mark points on the main survey line prior to taking offsets. This process continues until the data is recorded for all survey lines.
 
The levelling survey will begin by walking the site and deciding upon the size and position of the grid to be used to collect the height data. The surveyor will also make a note of the position of the TBM and which locations he might use for instrument positions to minimise the number of moves as shown in Figure 17 below.
  
 



Figure 17: Setting up a Grid
 
The grid is set out using steel tapes or bands to measure and ranging rods or arrows to mark each location on the grid. At this stage slope grid distances are used as these distances can be adjusted once the levels are known. A sketch showing the layout of the grid is recorded in the survey book prior to the commencement of the survey to allow clear identification of the spot levels and their location.
 
The optical level is set on the tripod and readings are taken on the TBM and grid points using the levelling staff. The readings are recorded in the levelling book as described in another unit.




Video /DVD



Websites


Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapters 4 & 7)


Self-Assessment Task

  • Explain the difference between an ordinary optical and automated piece of surveying equipment.









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