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Measurement System

Measurement System. PERSON. INSTRUMENT. ENVIRONMENT. SOURCES OF ERROR. Personal Errors. PERSON. INSTRUMENT. Instrument Errors. sight, touch limitations. Imperfections in Manufacturing or through use. ENVIRONMENT. Changes in temp, pressure, humidity, wind, etc. Natural

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Measurement System

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  1. Measurement System PERSON INSTRUMENT ENVIRONMENT

  2. SOURCES OF ERROR Personal Errors PERSON INSTRUMENT Instrument Errors sight, touch limitations Imperfections in Manufacturing or through use ENVIRONMENT Changes in temp, pressure, humidity, wind, etc. Natural Errors

  3. ERRORS VS MISTAKES Types of Errors: Systematic – conform to physical laws which can be modeled mathematically (e.g. expansion of a steel tape) Random - occur by chance, cannot be completely eliminated, but often compensate each other (+/-)\ - can be estimated using “least squares” Mistakes: These are blunders which occur from time to time in all measurement Errors are an unavoidable part of measurement and can mostly be adjusted out in survey computations, while mistakes must be detected before they lead to law suits…

  4. Dealing with Errors and Mistakes • Errors: • Errors can be minimized by:- • Calibrating equipment regularly • Following good observing practices that produce redundant • observations • Correcting for atmospheric variations • Mistakes: • Mistakes should be avoided at all costs, but are inevitable • and so we take independent observations to check that • measurements do not contain mistakes.

  5. ‘Polar’ Coordinates Line AB is a two-dimensional vector(α,L) defining the horizontal position of B relative to A. αAB denotes the azimuth from A to B LAB denotes the horizontal distance from A to B When AB is expressed in the form (α,L) it is referred to as the polar coordinates of B relative to A aAB LAB A(XA,YA)

  6. ‘Rectangular’ Coordinates Note that the vector AB can also be expressed as (∆X,∆Y) - referred to as the rectangular coordinates of B relative to A DXAB B(XB,YB) DYAB A(XA,YA)

  7. DXAB B(XB,YB) DYAB aAB LAB A(XA,YA) ‘Polar’ and ‘Rectangular’ Coordinates From Polar to Rectangular ∆XAB = LAB sin αAB ∆YAB = LAB cos αAB From Rectangular to Polar aAB = atanDXAB DYAB LAB = ((DXAB)2 + (DYAB)2)

  8. Compute Correct Azimuth in Inverse Problem arises because calculator or Excel does not know the correct quadrant of the line. This can be deduced by looking at the SIGN of DX and DY FIRST QUAD DX + DY + 2ND QUAD DX + DY - 4TH QUAD DX - DY + 3RD QUAD DX - DY -

  9. Compute Corrections to Atan 1st Quad: aAB = Atan (DX/DY) 2nd Quad:aAB = Atan (DX/DY) + 1800 3rd Quad: aAB = Atan (DX/DY) + 1800 4th Quad:aAB = Atan (DX/DY) + 3600

  10. TRAVERSING • Computations

  11. Traversing - Computations Traverse computations are concerned with deriving co-ordinates for the new points that were measured, along with some quantifiable measure for the accuracy of these positions. The co-ordinate system most commonly used is a grid based rectangular orthogonal system of eastings (X) and northings (Y). Traverse computations are cumulative in nature, starting from a fixed point or known line, and all of the other directions or positions determined from this reference.

  12. Traversing - Computations Angle/Bearing Computations and Balancing If angles are measured within a traverse, they need to be converted to bearings (relative to the meridian being used) in order to be used in the traverse computation. Before the bearings and azimuths are computed, the measured angles are checked for consistency and to detect any blunders.

  13. Traversing - Computations For closed traverses, a check can be applied to ensure that the measured angles can meet the required specifications. For a closed loop traverse with n internal angles, the check that is used is:  (internal angles) = (n – 2) 180 or (external angles) = (n + 2) 180 For a closed link traverse, the check is given by  A1 +(angles) – A2 = (n – 1) 180 where A1 is the initial or starting azimuth, A2 is the closing or final azimuth, and n is the number of angles measured.

  14. Traversing - Computations • The numerical difference between the computed checks and the measured sums is called the angular misclosure. There is usually a permissible or allowable limit for this misclosure, depending upon the accuracy requirements and specifications of the survey. A typical computation for the allowable misclosure  is given by • = kn where n is the number of angles measured and k is a fraction based on the least division of the theodolite scale. For example, if k is 1', for a traverse with 9 measured angles, the allowable misclosure is 3 '.

  15. Traversing - Computations Once the traverse angles are within allowable range, the remaining misclosure is distributed amongst the angles. This process is called balancing the angles : (i)  arbitrary adjustment – if misclosure is small, then it may be inserted into any angle arbitrarily (usually one that may be suspect). If no angle suspect, then it can be inserted into more than one angle. (ii) average adjustment – misclosure is divided by number of angles and correction inserted into all of the angles. (most common technique) (iii)  adjustment based on measuring conditions – if a line has particular obstruction that may have affected observations, misclosure may be divided and inserted into the two angles affected.

  16. Traversing - Computations • Errors in angular measurement are not related to the size of the angle. • Once the angles have been balanced, they can be used to compute the azimuths of the lines in the traverse. • Starting from the azimuth of the original fixed control line, the internal or clockwise measured angles are used to compute the forward azimuths of the new lines. • The azimuth of this line is then used to compute the azimuth of the next line and so on.

  17. Traversing - Computations The general formula that is used to compute the azimuths is:  forward azimuth of line = back azimuth of previous line + clockwise (internal) angle The back azimuth of a line is computed from  back azimuth = forward azimuth  180

  18. Traversing - Computations Therefore for a traverse from points 1 to 2 to 3 to 4 to 5, if the angles measured at 2, 3 and 4 are 100, 210, and 190 respectively, and the azimuth of the line from 1 to 2 is given as 160, then Az23 = Az21 + angle at 2 = (160 +180) + 100 = 440 80 Az34 = Az32 + angle at 3 = (80+180) +210 = 470110 Az45 = Az43 + angle at 4 = (110+180) +190 = 480120 210 100 1 3 190 4 2 5

  19. Traversing - Computations Once all of the azimuths have been computed, they can be checked and used for the co-ordinate computations.

  20. Co-ordinate Computations Traversing - Computations

  21. Traversing - Computations Alternative layout

  22. Traversing - Computations B 102° 11 ' A 172° 39 ' 104° 42 ' C 118° 34 ' 113° 05 ' 352° 39 ' E D

  23. Traversing - Computations Misclosures and Adjustments For closed traverses, since the co-ordinates of the final ending station are known, this provides a mathematical check on the computation of the co-ordinates for all of the other points. If the final computed eastings and northings are compared to the known eastings and northings for the closing station, then co-ordinate misclosures can be determined. The easting misclosure E is given by  E = final computed easting – final known easting similarly, the northing misclosure N is given by N = final computed northing – final known northing

  24. Traversing - Computations These discrepancies represent the difference on the ground between the position of the point computed from the observations and the known position of the point. The easting and northing misclosures are combined to give the linear misclosure of the traverse, where  linear misclosure = (E2 + N2) N E

  25. Traversing - Computations By itself the linear misclosure only gives a measure of how far the computed position is from the actual position (accuracy of the traverse measurements). Another parameter that is used to provide an indication of the relative accuracy of the traverse is the proportional linear misclosure. Here, the linear misclosure is divided by total distance measured, and this figure is expressed as a ratio e.g. 1 : 10000. In the example given, if the total distance measured along a traverse is 253.56m, and the linear misclosure is 0.01m, then the proportional linear misclosure is 0.01/253.56 = 1/25356 or approximately 1 : 25000

  26. Traversing - Computations The required accuracy of the survey in terms of its proportional linear misclosure also defines the equipment and allowable misclosure values. For example, for a traverse with an accuracy of better than 1/5000 would require a distance measurement technique better than 1/5000, and an angular error that is consistent with this figure. If the accuracy is restricted to 1/5000, then the maximum angular error is 1/5000 = tan  = 000'41" N  E

  27. Traversing - Computations The angular measurement for each angle should therefore be better than 000'41". The general relationship between the linear and angular error is given by the following table

  28. Traversing - Computations • The maximum allowable error in the traverse which is given by • = kn k therefore depends on the maximum allowable angular error as it relates to the least count of the instrument. For a 1/5000 traverse, the value of k = 30", so  = 30"n.

  29. Traversing - Computations If a misclosure exists, then the figure computed is not mathematically closed. This can be clearly illustrated with a closed loop traverse. The co-ordinates of a traverse are therefore adjusted for the purpose of providing a mathematically closed figure while at the same time yielding the best estimates for the horizontal positions for all of the traverse stations.

  30. Traversing - Computations • There are several methods that are used to adjust or balance traverses; • (i) Arbitrary method • (iii) Least-Squares • (iv) Transit rule • (v) Bowditch or Compass rule

  31. Traversing - Computations The arbitrary method is based upon the surveyor’s individual judgement considering the measurement conditions. The Least Squares method is a rigorous technique that is founded upon probabilistic theory. It requires an over-determined solution (redundant measurements) to compute the best estimated position for each of the traverse stations.

  32. Traversing - Computations The transit rule applies adjustments proportional to the size of the easting or northing component between two stations and the sum of the easting and northing differences. Therefore for two stations A and B, the correction to the easting and northings differences Eab and Nab are given by; correction to Eab = E(Eab/E) correction to Nab = N(Nab/N)

  33. Traversing - Computations In this method, if a line has no easting difference, then it will not have an easting correction, and similarly, if it has no northing difference there is no northing correction. Conversely, lines with larger easting and northing differences will have larger corrections. For example, consider a traverse that has an easting misclosure E of 0.170m and a northing misclosure N of 0.361m and the easting and northing differences are 54.439m and 1.230m respectively. If the sum of the easting differences is 587.463m and the sum of the northing differences is 672.835m, then  correction to Eab = 0.170(54.493/587.643) = 0.016m correction to Nab = 0.361(1.230/672.835) = 0.001m

  34. Traversing - Computations The Bowditch or Compass rule also applies a proportional adjustment, but in this case, the distances between the stations are used in proportion to the total distance of the traverse. The corrections are given by correction to Eab = E(distanceab/total distance of traverse) correction to Nab = N(distanceab/total distance of traverse

  35. Traversing - Computations This is the most commonly used technique for adjusting traverses. Using the above example, if the distance between A and B was 67.918m, and the total distance of the traverse was 1762.301m, then the corrections to be applied are correction to Eab = 0.170(67.918/1762.301) = 0.006m correction to Nab = 0.361(67.918/1762.301) = 0.014m

  36. Traversing - Computations 2.1. Blunder Detection Since traverse measurements involve angular and distance measurements, it is possible for blunders to exist in the measurements that are not detected until the final co-ordinate computations are made. Angular blunders manifest themselves in the angular closure and distance blunders in the co-ordinate closure, provided that the traverse is properly closed. In both cases it is possible to localise the blunder.

  37. Traversing - Computations To find an angular blunder, the traverse is computed without distributing the angular errors first in the forward direction, and then in the reverse direction. The point of intersection (where the co-ordinates are virtually the same) between the forward and reverse computations represents the location where the angular blunder was made, provided that only one blunder was made.

  38. Traversing - Computations A distance blunder causes a shift in the traverse section in the direction of the incorrect length. This is detected by checking the size and direction of the linear misclosure. If the linear misclosure is near a round figure (e.g. 1m or 5m) then a blunder probably exists within the measurements. The azimuth of the misclosure is then computed by Az = tan-1(E/N)

  39. Traversing - Computations If the azimuth is similar to any of the traverse legs, then it is likely that the distance blunder occurred when measuring this leg, and it can be corrected by remeasuring the line. If in the above example, the distance from A to B was measured as 75.11, then the resulting values for E andN would be –2.83m and –0.96m respectively. The azimuth of the misclosure would then be Az = tan-1(-2.83/-0.96) = tan-1 (2.9479) = 25115’

  40. Traversing – Computations This azimuth is almost the same as the azimuth of the line BA, so the distance blunder has been detected in this line. This method is limited when there are several legs with nearly the same azimuth. 

  41. Loop Traverse Check Interior Angle Closure ObservedAdjusted A =1010 24’ 00” 1010 24’ 12” B = 1490 13’ 00” 1490 13’ 00” C = 800 58’ 30” 800 58’ 30” D = 1160 19’ 00” 1160 19’ 00” E = 920 04’ 30” 920 04’ 30” Total = 5390 59’ 00” = 5400 00’ 00” Should = 5400 00’ 00” = (n-2)*180 Misclosure = 01’ 00” = 60” Adjustment = 60/5 = +12” per angle B 382.20 C N510 22’00”E 1490 13’00” 401.58 800 58’30” A 368.28 1010 24’00” 1160 19’00” 920 04’30” 350.10 D 579.03 E

  42. Compute Azimuths aAB = 510 22’ 00” aBA = 2310 22’ 00” B = 1490 13’ 12” aBC = 820 08’ 48” aCB = 2620 08’ 48” C = 800 58’ 42” aCD = 1810 10’ 06” aDC = 10 10’ 06” D = 1160 19’ 12” aDE = 2440 50’ 54” aED = 640 50’ 54” E = 920 04’ 42” aEA = 3320 46’ 12” aAE = 1520 46’ 12” A = 1010 24’ 12” aAB = 510 22’ 00” Check B C 510 22’00” 1490 13’12” 800 58’42” A 1010 24’12” 1160 19’12” 920 04’42” D E

  43. Compute Lats (D Cosa) and Deps (D Sina) Leg Azimuth Distance Lat Dep AB 510 22’ 00” 401.58 250.720 313.697 BC 820 08’ 48” 382.20 52.222 378.615 CD 1810 10’ 06” 368.28 -368.203 -7.509 DE 2440 50’ 54” 579.03 -246.097 -524.130 EA 3320 46’ 12” 350.10 311.301 -160.193 Total -0.057 0.480 Total Traverse Distance = 2081.19 Linear Misclosure = (0.057)2 + (0.480)2 = 0.483 Precision = 0.483/2081.19 = 1/4305 …… 1/4300

  44. Compass Rule/Bowditch Adjustment Correction to Lats = - Traverse leg distance * Lat Misclosure Total traverse distance Correction to Deps = - Traverse leg distance * Dep Misclosure Total traverse distance Leg Lats Deps Corrn Lat Corrn Dep Adj Lats Adj Deps AB 250.720 313.697 0.011 -0.093 250.731 313.604 BC 52.222 378.615 0.010 -0.088 52.232 378.527 CD -368.203 -7.509 0.010 -0.085 -368.193 -7.594 DE -246.097 -524.130 0.016 -0.134 -246.081 -524.264 EA 311.301 -160.193 0.010 -0.081 311.311 -160.274 Total 0.057 -0.481 0.000 0.001 Final Lats and Deps should be rounded to 2 decimal places

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