Chapter-2 Direct Measurement Indirect Measurement Method of linear measurement Taping corrections Optical measurement Tachometry EDM Measurement

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1 Chapter-2 Direct Measurement Indirect Measurement Method of linear measurement Taping corrections Optical measurement Tachometry EDM Measurement 1

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8 Methods There are 3 methods of making linear measurements. 1- Direct Method. 2- Optical Method. 3- E.D.M Method. 8

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31 Taping Corrections Incorrect length Slope Temperature Sag Stretch 31

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33 Slope 33 Trigonometry Horizontal: h = s*cos( ) Calculation s v h s v C h s h s h s v h s C 2 ) )( ( s h v

34 Slope Example If s = = 5 h = 300 cos(5) = v = 300 sin(5) = If you had measured v = C S = v 2 /2S = / = 1.14 h = v C S = = s h v 34

35 Temperature C t k( T T s )( L) k / F / C C t ( T T s )( L) 35

36 Temperature Example Tape calibrated to at 68 F Determine Dist AB = at 22 F Calculate true distance C T = (22-68)(368.50) = True Dist AB = =

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39 Sag and Tension C s C E p Steel w 2 L 3 W P 24P 2 L ( P Ps ) L AE 29,000,000 psi P 0.204W P Ps AE W = 2.8, A = 0.015, P S = 12 Trial and error -> P = 31 lb 39 If P = 18-lb, P S = 12-lb, L = 100, A = in 2, C P = (18 12)100/(0.015*29,000,000) =

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41 Taping Precision 1/2500 1/ Poor - Average 1/10,000 - Good 41

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49 Stadia is a tacheometric form of distance measurement that relies on a fixed-angle intercept while tacheometry is the procedure by which horizontal distances and difference in elevations are determined indirectly using subtended intervals and angles observed with a transit or theodolite on a graduated rod or scale. 49

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52 Example: HORIZONTAL SIGHTS 52

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54 Inclined Stadia Sights 54

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59 Example: INCLINED SIGHTS 59

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68 Measuring an angle Horizontal angles: use level, transit, or theodolite Vertical angle: use transit or theodolite Either: - graduated circles or - digital readout Both: Principles of Surveying. 2nd ed. C. A. Herubin,

69 Angles: readability Horizontal & vertical circles typically graduated to 1 o for construction grade instruments, 5 or better for survey instruments Vernier improves resolution by 10x or better Principles of Surveying. 2nd ed. C. A. Herubin, 1978 Digital readouts to 5 or better 69

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72 Electronic Distance Measurement Optical: uses parallax. Inexpensive but error 1% Ultrasonic: mid-priced. Accuracy ~ 0.1% Laser: moderate to very expensive. Accuracy 1 ppt or better 72

73 Measuring elevations Known as leveling Uses a level (optical or laser) & a rod All measurements are relative (to a starting elevation) Height of instrument 73 Both: Elements of Surveying. U. S. Army, TM 5-232, 1971

74 Optical vs. laser leveling Optical leveling requires 2 workers Laser leveling can be done alone, but easiest when rod is equipped with autodetector (high/low/on signals) Top: Principles of Surveying. 2nd ed. C. A. Herubin, 1978 Bottom: Topcon web site 74

75 Putting it together Two ways of mapping a region: Traversing used to locate specific features Triangulation used to establish a control network over a region 75 Both: Elements of Surveying. U. S. Army, TM 5-232, 1971

76 Types of traverses Allowable misclosure First order, Class I: - 4 mm in 1 km mm in 1000 km Third order: - 12 mm in 1 km mm in 1000 km Land surveys:??? 76 Both: Elements of Surveying. U. S. Army, TM 5-232, 1971

77 The total station Combines theodolite, EDM, data logger & surveying software Log ~ 8000 points, download data to computer Why doesn t ES have one??? 77 Topcon web site

78 Electronic Distance Measurement (EDM): is an instrument that transmits a carrier signal of electromagnetic energy from its position to a receiver located at another position Quick and precise measurements. Save time and money Automatically display direct readout measurements. Mistakes are reduced EDM instruments are combined with digital theodolites and microprocessors to produce total station instruments. Precise taping is one of the most difficult and painstaking of all surveying tasks. 78

79 Basic Instruments An electronic distance-measuring devise A reflector consists of several prisms mounted on a tripod From EDM To EDM Measuring unit or transmitter Reflector 79

80 Types of EDMs Based on the wavelengths of the electromagnetic energy which they transmit, there are two types of EDMs. 1. Electro-optical instruments: transmit light in short wavelengths of about 0.4 to 1.2 µm. (laser and infrared) This light is visible or just above the visible (laser and infrared). Almost all short-range EDMs for measuring up to a few miles are of the infrared type. Laser type are visible 2. Microwave instruments: Transmit long wavelengths of about 10 to 100 µm. The waves penetrate through fog or rain More affected by humidity than are the light-wave instruments. 80

81 Wavelength a) Longer wavelength b) Shorter wavelength 81

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83 Principles of electronic distance measurement Distance are observed electronically by determine the number of full and partial waves. Knowing the precise length of the wave, the distance can be determined. 83

84 procedure for measuring 1. The EDM device is set up, centred, and levelled at one end of the line. 2. The prism assembly is placed at the other end of the line 3. The telescope is sighted toward the prism and the power is turned on. 4. The instrument transmits a signal to the reflector. 5. The reflector returns the signal to the receiver, so it its travel path is double the distance. 6. The EDM device determines the number of waves in the double path, multiplied by the wavelength, and divided by 2 to obtain the distance. 84

85 Some fractional part of the wavelength would in general be expected, therefore the distance between the EDM instrument and reflector is expressed as: L n 2 p Where L distance between the EDM and the reflector. λ is the wave length n is the number of full wavelength p the length of the fractional part. 85

86 The fractional length is determine by the EDM instrument from measurement of phase angle of the returned signal. λ / 2 λ 0.375λ 0 o 135 o 90 o 180 o 270 o 360 o 86 One cycle A wave length of electromagnetic energy illustrating phase angle

87 Example: Assume that a wavelength is precisely m. Assume also that the number of full waves is 9 and phase angle of the returned signal is o determine the length of the distance being measured. L n 2 p Length p would be (115.7/360) x =6.428 m L m 2

88 Errors in the EDM measurements 1. Personal errors Not Setting the instrument or reflector exactly over the point. Not measuring the instrument height and weather conditions perfectly. 2. Natural errors Variation in temperature, humidity, and pressure. For electro-optical instruments, correction for humidity can be neglected. Snow, fog, rain, and dust affect the visibility factor for EDMs. 3. Instrumental Errors Very small if the equipment has been carefully adjusted and calibrated. 88

INTRODUCTION TO ENGINEERING SURVEYING (CE 1305)

INTRODUCTION TO ENGINEERING SURVEYING (CE 1305) Linear Measurements Sr Dr. Tan Liat Choon Email: tanliatchoon@gmail.com Mobile: 016-4975551 1 EDM, TOTAL STATION, PRISM AND POLE 2 ELECTROMAGNETIC DISTANCE