SURVEY2
UNIT- 2Geodetic Surveying Q1) What is reduction to centre and triangulation network?A1) Reduction to centreIt is defined as the process of computing the correct angle from observed angle is called reduction to centre.Triangulation network: -It is a network of of triangles used to find the distance and position of point.Network is done by measuring length of one side of a triangle and reducing its angle and length of other two sides by observation from baseline Q2) Explain signals and its types?A2) Signals: - - It is small device which is used to define location of triangulation station such that it is easily observed from other stations.- Signal must be vertical over station.- It is placed centrally over station mark.- It is important to place over station mark because accuracy of triangulation system depends on signal that is entering of signal.Signal: -1) Opaque signal2) Luminous signala) Opaque signal: -Is type of signal used where less accuracy and for short sight distance (i.e. not more than 30 kilometer)It must be used in day time only.Cheaper than luminous signal.Example of opaque signal(i)Target signal – suitable for distance upto 30 km(ii) Pole signal- distance up to 6 km(iii) Elevated signal – elevated tower are used as signal and suitable for distance upto 30 to 40 km.(iv)Stone signpost(v)Pole and brush signalb) Luminous signal: -This type of signal basically used for geodetic survey because of quite distinct and have clear visibility even for long distance (more than 30 km).It is used for day as well as night also.Luminous signal-Sun signal-Light signal(i) Sun signal - also known as heliotrope. Heliotrope reflect sun rays towards station.(ii) Night signals - observation is at night. Eg. Drummond's light and magnesium lamp with parabolic reflector Q3) What is the eror in substance bar?A3) a. Instrumental(1) Geometry(a) PrincipleThe triangle formed by the subtense bar and instrument is a long narrow isosceles triangle. The angle measured at the instrument is small making for some weak geometry, particularly in long distances. (b) Behavior (c) Compensation (2) Orientation(a) PrincipleDistance determination is predicated on the subtense bar being perpendicular to the line. Most had a sighting system which helped orient the bar correctly
(b) BehaviorMisorientation of the bar introduces a systematic error, Figure
Without knowing the rotation angle and its direction, however, it's impossible compute the corrections. Even if the rotation were known, the horizontal angle corrections would be small. For example, at 100 meters, a 2-meter-long bar rotated 10° results in a horizontal angle error of 0°00'07". This would be well inside pointing and setup errors. (c) CompensationExperience; Careful equipment set up.b. Natural(1) Refraction and Curvature(a) PrincipleThe line of sight, on which subtense bar use depends, is bent by the atmosphere and deviates from a level line due to earth curvature. The effect of each increases as does distance. However, the line of sight to each end of the bar deviates the same amount and in the vertical direction so the combined effect does not affect the horizontal angle measurement.At a distance of 1000 ft (pushing it with a subtense bar) and a 65°00'00" zenith angle (pretty steep), varying the angle 4.9" makes a difference of 0.01 ft horizontal and 0.02 ft vertical, neither of which affects the distances.For all intents and purposes, curvature and refraction don't affect subtense bar measurements.(b) BehaviorSystematic but negligible.(c) CompensationNot necessary(2) Atmospheric conditions(a) PrincipleIn addition to refraction, local atmospheric anomalies can have varying effect when the line of sight passes near a radiant surface which reflects direct sunlight pr emits absorbed heat. The line of sight in such conditions is not stable which introduces pointing errors.(b) BehaviorRandom(c) CompensationSelect locations either removed from heat wave sources or limit observations to time of day (early) before the sun heats the intermediate surface or absorption takes place.(3) Temperature(a) PrincipleSubtense bars are constructed of low thermal expansion alloy materials. They are not affected by temperature changes sufficiently to alter distance measurement within useful operating range.Temperature is more an instrument operator comfort issue.(b) BehaviorSystematic but negligible.(c) CompensationNot necessary.c. Personal(1) Set up(a) PrincipleBoth the subtense bar and measuring instrument bust be set up over their respective points. This is more fully discussed in the Electronic Distance Measurement section.(b) BehaviorRandom(c) CompensationCareful set ups with practice will minimize the random error.(2) Pointing(a) PrincipleThe surveyor must sight both targets as accurately as possible. Longer distances are increasingly affected as the overall angle between subtense target gets smaller.(b) Behavior (c) Compensation (3) Computations(a) Behavior (b) Compensation Q4) Define substance bar?A4) A subtense bar (fig. is a horizontal staff with targets fixed at a known distance apart. It is about 4m long having a small spirit level and a quick levelling head. A sight rule, provided at its centre, can be placed along line of sight by viewing the telescope of the theodolite thought the vanes. The bar is mounted on a tripod and is placed at right angles to line of sight for making observations. After levelling and aligning, it is clamped by means of clamp screw. The targets, made of discs of about 20 cm diameter are painted red on one side, and white on the other. The centres of body sides of the targets are painted black in 7.5 cm diameter. The targets are placed at a distance of 2.5 m and 3 m. When targets are placed 2.5 m apart, the white faces are to face instrument and when they are placed 3m apart, the red faces face instrument.
The horizontal and vertical angles are measured with a transit theodolite Q5) What is baseline?A5) Baseline is a line between two points on the earth's surface and the direction and distance between them. A baseline is a line that is a base for measurement or for construction Q6) What are the instrument used in baseline?A6) 1) Tape2) Straining device3) Weight and pulley4) Thermometer and finely divided scale.5) Marking tripod or station rakes6) Supporting tripods7) Chains Q7) What are the base line correction in survey?A7) Chain / Tape corrections1)Correction due to standardizationCorrection per chain length
Correction due to slope
1) Correction due to wrong alignment
Q8) How to choice baseline?A8) Choices of Site selection for baselineFollowing factors must be accounted for while selecting the site for baselinea. Ground should be firm and smoothb. Two ends of baseline must be intervisible.c. Site must be free from any obstruction throughout base length.d. Site should be well shaped triangle can be obtained while connecting it’s and stations to main triangulation station.e. Site should be such that less length of baseline as specified is available.f. Site must be level. Q9) What is signal in triangulation?A9)- It is small device which is used to define location of triangulation station such that it is easily observed from other stations.- Signal must be vertical over station.- It is placed centrally over station mark. Q10) What do you mean by satellite station? A10) It is also known as eccentric station.Satellite station is a false station because it is not possible to set the instrument over the structure false station point is assumed.
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Total correction
True length of line Alternate solution of Q →(1) Core (1) L = 30m andL' = 30.40 m 1) C' = L'-L = 30.40-30 = (+) 0.40m 2) Number of chains = n = 3) Total correction
4) Total length l = 12690+169.20 = 16459.2 m |
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This correction is always a negative correction. - Correct distance between any two points on ground is horizontal distance. Correction required = L –
If
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Same as slope correction If endpoint of chain is not put on the straight line between points to be measured
Correction is always negative. 4)Temperature correction If
L = length of chain / tape
1) Pull correction
L = length of chain / tape
E = Young's modulus Pull correction =
2) Sag correction
This collection is always negative. Formal tension The value of pull For which positive pull correction is exactly equal to negative sag correction is called normal tension.
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