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ERT252 GEOMATIC ENGINEERING

ERT252 GEOMATIC ENGINEERING. MRS SITI KAMARIAH MD SA’AT 019-5706232 sitikamariah@unimap.edu.my. LINEAR MEASUREMENT. Introduction. One of the fundamentals of surveying is the need to measure distance . Distances are not necessarily linear, especially if they occur on the spherical earth.

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ERT252 GEOMATIC ENGINEERING

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  1. ERT252 GEOMATIC ENGINEERING MRS SITI KAMARIAH MD SA’AT 019-5706232 sitikamariah@unimap.edu.my

  2. LINEAR MEASUREMENT

  3. Introduction One of the fundamentals of surveying is the need to measure distance. Distances are not necessarily linear, especially if they occur on the spherical earth. In this course we will deal with distances in geometric space, which we can consider a straight line from one point or feature to another.

  4. Distance Measurements • Distance between two points can be horizontally, slopeor vertically recorded in feet/meters. • Horizontal and slope distance can be measured using fibreglass tape/steel tape/using electronic distance measuring device. • Vertical distance can be measured using a tape, as in construction work, with a autolevel and staff. It also can be determine by trigonometry.

  5. Slope, Vertical and Horizontal Distances

  6. Distance Measurements

  7. Distance Measurement Equipment

  8. ELECTRONIC DISTANCE MEASUREMENT (EDM) EDM is very useful in measuring distances that are difficult to access or long distances. It measures the time required for a wave to sent to a target and reflect back.

  9. Taping (or chaining) • Taping is applied to measurement with a steel tape or synthetic tape (plastic or fiberglass). • All standard in lengths • 100 m, 50m, 30 m, 20 m. • It is fairly quick, easy and cheap, and hence is the most common form of distance measurement.

  10. Taping (or chaining) Unfortunately, taping is prone to errors and mistakes. For high accuracy, steel tape should be used which is graduated in mm and calibrated under standard temp (20 degree) and tension (5kg). Be careful, easily break. Synthetic tape is more flexible graduated in 10mm

  11. Taping

  12. Taping on smooth level/sloping ground Tape must always be straight Tape must not be twisted Use chaining arrows for intermediate points Tape horizontally if possible Tape on the ground if possible Slope taping needs to be reduced Catenary taping requires correction Step taping suits some applications

  13. distance distance required measured Tape must be straight… obstruction measured distancerequired distance

  14. For very long distance

  15. For very long distance Length AB = 4 x Full tape distance + 1 Short section REMEMBER ! It works only on smooth ground or uniform slope surfaces

  16. distance distance measured required Use chaining arrows… measured distancerequired distance

  17. Sloping Ground Measurement

  18. Slope Measurement

  19. Sources of Error in Taping • Instrumental errors • actual length can be different from nominal length because of a defect in manufacture or repair on as a result of kinks. • Natural errors • the horizontal distance of a tape varies because of effects of temperature, wind and weight of tape itself. • Personal errors • Tape persons may be careless in setting pins, reading tape or manupulating equipment.

  20. Taping Errors • Typical taping errors: • Incorrect length of tape • Temperature other than standard

  21. Taping Corrections For synthetic tapes, only Standardized Tape Length correction and Slope corrections will be applied The best accuracy that can be achieved is the order of 1:1000 When using steel tapes, if only Standardized Tape Length and slope corrections are considered, the best possible accuracy that can be obtained in the range 1:5000. If tension and temperature are added into consideration, accuracy can be increased to better than 1:10000 ~ 1: 20000 Sag only applies if tape is supported only at ends

  22. Standard Length Correction Where Ca = correction of absolute length C = correction to be applied to the tape L= measured length l =nominal length of the tape Example: A distance of 220.450 m was measured with a steel band of nominal length 30 m. On standardization the tape was found to be 30.003 m. Calculate the correct measured distance, assuming the error is evenly distributed throughout the tape. Error per 30 m, C = 3 mm Correction for total length = = 220.45(0.003)/30 =0.022 m Correct length is 220.450 + 0.022 = 220.472 m

  23. Slope Correction Consider a 50-m tape measuring on a slope with a difference in height of 5 m. The correction for slope is = –25/100 = –0.250 m

  24. Tension correction E is modulus of elasticity of tape in N/mm2 = 2.1 x 105 N/mm2 for steel A is cross-sectional area of the tape in mm2 L is measured length in m; and Po is the standard pull P is pull applied during measurement As the tape is stretched under the extra tension, the correction is positive. If less than standard, the correction is negative.

  25. Tension correction Consider a 50-m tape with a cross-sectional area of 4 mm2, a standard tension of 50 N and a value for the modulus of elasticity of E = 210 kN/mm2. Under a pull of 90 N the tape would stretch by

  26. Temperature Correction Where Tm = mean temperature during measurement To = temperature of standardization Coefficient of expansion of steel α = 3.5 × 10–6 per oC If L = 50 m and the different of temperature and standard temperature (20oC) in temperature is ± 2°C then =?

  27. Sag Correction Consider a 50-m heavy tape of w = 1.7 kg with a standard tension of 80 N:

  28. Summary : Correction Slope correction Standardization correction Tension correction Temperature correction Sag correction

  29. Example 1: A steel tape of nominal length 30 m was used to check the distance between two offset pegs A1 and A2. The following results were obtained. Compared to 25.000 m baseline, the tape read 24.994 m with 50 N tensions applied to at 15oC. The cross sectional area of the tape is 2.0 mm2 and it weigh 4.5 N. Calculate the horizontal length A1 to A2.

  30. Solution Slope correction slope correction = – 0.0851 m Standardization correction standardisation correction = + 0.0056 m Tension correction tension correction = + 0.0029 m Temperature correction Sag correction Horizontal length A1 to A2 =

  31. ELECTRONIC DISTANCE MEASUREMENT (EDM) EDM is very useful in measuring distances that are difficult to access or long distances. It measures the time required for a wave to sent to a target and reflect back.

  32. Principles of EDM Operation: A wave is transmitted and the returning wave is measured to find the distance traveled. V= f λ

  33. General Principle of EDM • Electromagnetic energy • Travels based on following relation: • Intensity modulate EM energy to specific frequency • Distances determined by calculating the number of wavelengths traveled.

  34. Principles of EDM • Errors are generally small and insignificant for short distances. • For longer distances they can be more important. • Errors can be accounted for manually, or by the EDM if it has the capability.

  35. EDM Classifications • Described by form of electromagnetic energy. • First instruments were primarily microwave (1947) • Present instruments are some form of light, i.e. laser or near-infrared lights. • Described by range of operation. • Generally microwave are 30 - 50 km range. (med) • Developed in the early 70’s, and were used for control surveys. • Light EDM’s generally 3 - 5 km range. (short) • Used in engineering and construction

  36. EDM Properties Total station = Theodolite with built in EDM + Microprocessor They come in long (10-20 km), medium (3-10 km), and short range (.5-3 km). Range limits up to 50 km They are typically mounted on top of a theodolite, but can be mounted directly to a tribrach.

  37. EDM Characteristics 750-1000 meters range Accurate to ±5mm + 5 ppm Operating temperature between -20 to +50 degrees centigrade 1.5 seconds typical for computing a distanc, 1 second when tracking. Slope reduction either manual or automatic. Some average repeated measurements. Signal attenuation. battery operated and can perform between 350 and 1400 measurements.

  38. EDM Accessories

  39. Uses • Total stations are ideal for collecting large numbers of points. • They are commonly used for all aspects of modern surveying. Only when harsh conditions, exist or distances are short will a transit and tape be used.

  40. The Total Station Measures and Records: Horizontal Angles Vertical Angles and Slope Distances Calculates: Horizontal Distance Vertical Distance Azimuths of Lines X,Y,Z Coordinates Layout Etc.

  41. Prism Made from cube corners Have the property of reflecting rays back precisely in the same direction. They can be tribrach-mounted and centered with an optical plummet, or they can be attached to a range pole and held vertical on a point with the aid of a bulls-eye level.

  42. Observation using EDM

  43. Sources of Errors in EDM • Personal: • Careless centering of instrument and/or reflector • Faulty temperature and pressure measurements • Incorrect input of T and p • Instrumental • Instrument not calibrated • Electrical center • Prism Constant (see next • slide) • Natural • Varying ‘met’ along line • Turbulence in air

  44. Systematic Errors • Microwave • Atmospheric conditions • Temperature • Pressure • Humidity - must have wet bulb and dry bulb temperature. • Multi-path • Reflected signals can give longer distances

  45. Systematic Errors • Light • Atmospheric conditions • Temperature • Pressure • Prism offset • Point of measurement is generally behind the plumb line. • Today usually standardized as 30mm.

  46. Instrumental Errors o-------------------------------o-------------o A B C • Measure AB, BC and AC • AC + K = (AB + K) + (BC + K) • K = AC- (AB + BC) • If electrical center is calibrated, K represents the prism constant. Both the prism and EDM should be corrected for off-center characteristics. The prism/instrument constant (about 30 to 40 mm) can be measured by measure AC, AB, and BC and then constant = AC-AB-BC

  47. Problems using EDM Total stations are dependant on batteries and electronics. The LCD screen does not work well when it is cold . Battery life is also short, batteries and electronics both do not work well when wet. Total stations are typically heavier that a transit and tape Loss of data is an important consideration

  48. COMPUTATION OF HORIZONTAL LENGTH FROM SLOPE DISTANCE USING EDM

  49. By Elevation Differences H = √(L2-d2) d=(elevation A + he) – (elevation B + hr)

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