Week1 introductiontobasicsurveying


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Week1 introductiontobasicsurveying

  1. 1. A2.2NP1 Environmental Practical 1 TOPIC 1 TECHNIQUES IN BASIC SURVEYING
  2. 2. Basic ideas • Surveying - the creation of a scale representation of the ground surface - is a basic activity in many areas of environmental management. • A survey will be one of of two types: – Primary survey - to establish the position of objects in three dimensions when no previous information exists – Secondary survey - to add extra information to existing data or to measure changes over an interval of time
  3. 3. Basic ideas • The task of three dimensional position fixing is normally broken into two parts: • Determining plan position • Determining elevation
  4. 4. Basic ideas • Each of these determinations may be either: • absolute - made in terms of a fixed co-ordinate system • relative - made in terms of local co-ordinates which may later be converted to absolute co- ordinates if required. • The majority of surveys carried out for environmental management are thus secondary relative surveys
  5. 5. Plan Position Fixing • The plan position of a station can be established in a number of ways:
  6. 6. • By reference to the apparent positions of astronomical objects when viewed from that station • This method gives the absolute location of the station in terms of latitude and longitude, which can be converted to local systems such as the National Grid.
  7. 7. • By the measurement of the angles between lines of sight to the unknown station from other known positions • By the intersection of lines of sight from the unknown station to other objects whose positions are already known • These two methods both rely on the simple Euclidean geometry of the plane. (Hence the term plane surveying). The first procedure is termed triangulation and the second resection.
  8. 8. BaselineA B The basic principle of triangulation Measured angle Measured angle
  9. 9. Unknown position The basic principle of resection Measured angle Measured angle Known position Known position Known position Measured angle
  10. 10. • By measurement of distances between the unknown station and other objects of known positions • This last method includes a number of particular cases:
  11. 11. • measurements of offset distances from a base line. • trilateration - the distance equivalent of triangulation. • tacheometry - an optical method of distance measurement along a known bearing
  12. 12. BaselineA B The basic principle of trilateration Measured side Measured side
  13. 13. Plane Surveying: Theory • Plane surveying relies on the basic concepts of Euclidean geometry, and in particular the properties of triangles. • The most important (for our purposes) of these are:
  14. 14. Plane Surveying: Theory • The internal angles of a triangle sum to 180° • The sides of an equilateral triangle are equal and the internal angles are all 60° • The base angles and opposing sides of an isosceles triangle are equal
  15. 15. 60º 60º 60º The equilateral triangle All sides equal in length All angles equal (= 60º)
  16. 16. The isosceles triangle Two sides equal in length Two angles equal a a
  17. 17. Plane Surveying: Theory • If the respective angles in two triangles are equal then the triangles are similar and their sides are all in the same proportion • If two triangles have two angles and one side equal (or vice versa) then they are congruent and all their other respective angles and sides are equal. • Two triangles are also congruent if all their sides are equal.
  18. 18. Similar triangles have corresponding angles equal but are of differing size
  19. 19. Conguent triangles are identical • two angles and one side equal • two sides and one angle equal • all three sides equal
  20. 20. Plane Surveying: Theory • Congruent triangles are unique - you cannot draw two different triangles from the same set of measurements • This means that a complete set of survey data must define the positions of objects uniquely.
  21. 21. Plane Surveying: Theory • Any closed polygon can be subdivided into a series of contiguous triangles • These properties are repeatedly used in the procedure of triangulation in which stations are surveyed in a pattern of contiguous triangles.
  22. 22. Any closed polygon can be subdivided into contiguous triangles These should be chosen to make as many of the triangles as close to equilateral as possible
  23. 23. Plane surveying: practical aspects • In practice, most plane surveys are carried out in a straightforward way following an established sequence: 1. A reconnaisance survey will establish the dimensions of the area, relative levels, significant features, accessibility, obstacles etc
  24. 24. Plane surveying: practical aspects 2. Establish an accurate baseline by measurement from existing survey points, natural features, buildings etc. If none are available then the baseline must be fixed by absolute methods. 3. Establish as required any further control points by triangulation or trilateration from the base-line.
  25. 25. Plane surveying: practical aspects 4. Incorporate detail by tacheometry, traversing, tape & offset or whatever other method is appropriate. 5. The intermediate stations should where appropriate be cross-checked with the control points by resection and all traverses should be closed at a control point. 6. Inaccessible detail should be incorporated by triangulation or plane tabling from the ends of the baseline.
  27. 27. Plane surveying: practical aspects 7. If a topographic survey is being undertaken, levelling traverses should be carried out around the survey stations and the baseline tied to the local benchmark by a closed traverse. 8. The use of a theodolite or total station will enable both the position and the elevation of stations to be found simultaneously by combined tacheometry and triangulation or by trilateration
  28. 28. THE “CHAIN” SURVEY How to establish relative plan positions
  29. 29. Chain survey • Simplest of all survey techniques • Relies on linear measurements; slopes >3o require some adjustment to technique • Usually requires a clear line of sight • The triangles used should be equilateral or approximately so
  30. 30. Terminology • Trilateration is the measurement of sides of a triangle • whereas triangulation refers to the measurement of the angles of the triangle
  31. 31. Basic equipment • Ranging poles • Survey pegs and ‘arrows’ • Chain & tape measure or other distance measuring instrument • Plumb line • Compass
  32. 32. Chain survey components • Base line: the longest line • Chain /survey lines • Survey stations • Offset lines
  33. 33. Order of events • “Range out” survey stations with ranging rods • Establish base line and measure accurately • Measure remaining distances between other survey stations • Measure offset lines whilst measuring between survey stations
  34. 34. Sloping ground • If the ground slopes by more than about 3°, this must be allowed for in the survey. • The measured distances are thus slant distances and must be corrected to true horizontal distances. • This requires that the vertical angle between the stations is known
  35. 35. Ground distance determined a h X
  36. 36. Sloping ground • For an approximate survey, it may be sufficient to step up or downhill using a series of horizontal and vertical lines • If the drop is measured at the same time, some estimate of the slope profile can be obtained
  37. 37. Chain surveying (“stepping”) w x y z c b a
  38. 38. Sloping ground • If stepping is not appropriate, more sophisticated methods must be used to measure the slant distance and the vertical angle simultaneously • Requires optical sighting equipment: usually either a clinometer, Abney level or theodolite
  39. 39. Basic levelling in chain surveys a h h
  40. 40. Correcting for horizontal distance: the “hypotenusal allowance” a h z correction factor = xy - yz = xy(1 - cosa) y x
  41. 41. LEVELLING How to destermine relative elevations
  42. 42. Levelling: accounting for slopes Unlike chain surveys, levelling surveys account directly for slope and incorporate this data into the whole measurement exercise AIMS: • to determine height differences between two points • to determine elevations for sections
  43. 43. • The elevation of a station can be established by: • inclined line of sight from chain survey stations • levelling from another point of known height • by inclined tacheometry
  44. 44. • Levelling is the more accurate method but is also the slower. Modern instruments are capable of cm accuracy under normal conditions over distances of 100’s metres. • The keys to successful levelling lie in the setting up of the instrument, in the closure of the traverses and in the careful recording (booking) of the results.
  45. 45. • Inclined tacheometry relies on the combined measurement, by theodolite, of the slant distance to the new station and the angle relative to the horizontal. • The elevation change and horizontal distance can then be found by simple trigonometry. • The accuracy of the method, using normal instruments, is around 10’s cms in 100’s metres.
  46. 46. Direct levelling • Most typical form used Relies upon: • a horizontal line of sight, also termed “the line of collimation” • a fixed datum level
  47. 47. Measurements to be taken • Backsight • Foresight • Intermediate sights
  48. 48. Booking your results The “rise and fall” method • This method records the relative change in level between successive stations • The changes are converted to the reduced level of each station • The reduced level is relative to the local datum
  49. 49. Booking the results • The method relies on recording your results in a survey book in a standard format • This allows you to check your work and to identify any errors systematically
  50. 50. Reduced levels The change of level is 2.312m - 2.533m = -0.221m 2.533m Datum line: 100.522m (from OS Benchmark) 2.312 m The reduced level of point B is 100.301m B A The absolute (datum) level of point A is 100.522m IP 1
  51. 51. Backsight Interm. Foresight Rise Fall R.L. Distance Remarks 2.312 100.522 0.221 100.301 1.2 Rise and fall booking Point A Point B - 2.533
  52. 52. Transfer of level The new change of level is 1.674m - 1.631m = + 0.043m 1.631m 1.674 m The absolute level of point C is 100.344m C B At the next stage, B becomes the backsight and C is the new foresight IP 2
  53. 53. Backsight Interm. Foresight Rise Fall R.L. Distance Remarks 2.312 100.522 - 0.221 100.301 1.2 Rise and fall booking (cont) Point A Point B - 2.5331.674 1.631 + 0.043 100.344 Point C
  54. 54. • Continuing this process, suppose we end up with a set of results as follows: • This will enable us to check our working
  55. 55. Backsight Interm. Foresight Rise Fall R.L. Distance Remarks 2.312 100.522 - 0.221 100.301 Rise and fall booking (cont) Point A Point B -- 2.5331.674 1.631 + 0.043 100.344 Point C2.504 3.010 2.413 0.956 2.016 2.718 -- -- -- -- + 1.548 + 0.994 -0.305 101.892 102.886 -- -- -- -- 102.581 11.913 9.854 9.854 2.585 - 0.526 - 0.526 102.581 - 100.522 2.059 2.059 2.059 CHECKS OK
  56. 56. Using an intermediate sight • Sometimes we wish to include a specific feature but it is not convenient to set up a new instrument position for this • The solution is to take a sighting onto the staff when it is placed on this feature - this is called an intermediate sight
  57. 57. Intermediate sight The new change of level is 1.674m - 2.988m = -1.314m 2.988m The absolute level of the intermediate point C is 98.987m C B The intermediate sight is taken at the base of the channel between B and C IP 2 Intermediate sight 1.674m
  58. 58. Backsight Interm. Foresight Rise Fall R.L. Distance Remarks 2.312 100.522 - 0.221 100.301 1.2 Rise and fall booking (intermediate sight) Point A Point B - 2.5331.674 1.631 + 0.043 100.344 Point C 2.988 channel-1.314 98.987 Next FS
  59. 59. Optical distance measurement • It is often convenient to use the levelling instrument itself to calculate the distance between the instrument and staff positions • This is done using the stadia lines that are visible in the viewfinder • These are arranged such that the distance to the staff is 100x the stadia interval that is read on the staff between the two lines • This procedure is known as tacheometry
  60. 60. Tacheometry The viewfinder: Stadia lines Multiply vertical distance by 100 to obtain horizontal distance
  61. 61. Inclined tacheometry • If the ‘level’ can be swung in a vertical arc, the distance up an inclined sight line can be obtained. • If the vertical angle is also measured, the slant distance can be converted to give both the change in height and the true horizontal distance.
  62. 62. Inclined tacheometry a Change of height Tacheometric distance Measured angle True horizontal distance
  63. 63. The theodolite • If such an instrument can also be swung in a horizontal arc, and the angle of rotation can be measured, we are able to determine the angles of the sight lines between stations. • This allows both trilateration and triangulation with the same instrument. • Such a versatile instrument exist and is called a theodolite.
  64. 64. Summary • Chain surveys are suited to planimetric surveys on low slopes. They rely upon trilateration. • Levelling is used where terrain is more uneven. Levelling surveys often use tacheometry to fix station positions. • A theodolite survey permits levelling, tacheometry or triangulation as required.