Tacheometric surveying ppt

Advanced Surveying,I scheme,MSBTE

1. UNIT 3 TACHEOMETRIC SURVEYING ( 8 MARKS)

2. COURSE OUTCOMES C . Find distances and elevation using tacheometer

3. PRACTICAL OUTCOMES 14. Use theodolite as a tacheometer to compute reduced levels and horizontal distances

4. UNIT OUTCOMES 3a. Explain the functions of given component(s) of tacheometer 3d. Determine RLs of stations and distances between stations using tacheometric survey using given data 3c.Calculate constant of tacheometer from given data 3b.Determine horizontal and vertical distances using tacheometric formula in the given situation

5. Introduction Tacheometric is a branch of surveying in which horizontal and vertical distances are determined by taking angular observation with an instrument known as a tachometer. Tacheometric surveying is adopted in rough and difficult terrain where direct leveling and chaining are either not possible or very tedious.

6. Suitability/uses P reparation of topographic map where both horizontal and vertical distances are required to be measured S urvey work in difficult terrain where direct methods of measurements are inconvenient Establishment of secondary control points reconnaissance survey for highways and railways etc.

7. Instruments used for tacheometric surveying Tacheometer Levelling and stadia staff rod

8. Tacheometer A tacheometer is similar to an ordinary transit theodolite, generally a vernier theodolite itself, fitted with two stadia wires in addition to the central cross-hair and anallatic lens.

9. Characteristics of Tacheometer

10. Stadia diaphragm Upper hair Lower hair Middle hair

11. Levelling or stadia staff rod For short distances, ordinary leveling staves are used. The leveling staff normally 4m long, and it can be folded with here parts. The graduations are so marked that a minimum reading of 0.005 or 0.001m can be taken.

12. Anallatic lens   Anallatic lens   is an additional lens used in the instrument. It is a special lens which is placed between the object glass and the eyepiece of the telescope in order to eliminate the additive constant ( f+c). This is done to make the expression for the distance between instrument station and staff position more simplified. The lens in only provided in an external focusing telescope but not in the internal focusing

13. Principle of tacheometry Principle of tacheometry is based on principle of similar isosceles triangle in which corresponding sides & altitudes are proportional The ratio of distance of base from apex and length of base is always constant.

14. Principle of tacheometry In fig. Oa1a2, Ob 1 b 2 , Oc 1 c 2 are all isosceles triangles where D 1 , D 2 , D 3 are the distances of bases from the apices (distances of staff stations from instrument stations) and S 1 , S 2, S 3 are the lengths of the bases also called staff intercepts. According to stated principle. D1/S 1 = D2/S 2 =D 3 / S 3 =f/ i =Constant Where f=focal length of object and i =stadia intercept D1 D2 D3

15. Methods of tacheometry

16. Tacheometric Formula Q B A b c a Instrument Axis Staff Objective Focus O Diaphragm f c i s C d’ d D= (f/ i ) s+ (f+c) D

17. Tacheometric Formula O is the optical centre of the object glass f = focal length of the object glass , i = stadia hair interval = ab , s = staff intercept = AB , c = distance from O to the vertical axis of the instrument , d = distance from O to the staff , d ′ = distance from O to the plane of the diaphragm, and D = horizontal distance from the vertical axis to the staff.

18. Tacheometric Formula When line of sight inclined and staff held vertical D= (f/ i ) s cos 2 θ +(f+c)cos θ L D

19. Tacheometric Formula When line of sight inclined and staff held vertical V = (f/ i ) s cos θsin θ +( f+c)sin θ Denoting QC , the central hair reading as h , the level difference between G and Q for an angle of elevation is given by FQ = V − h and if θ is angle of depression, FQ = V + h

20. Now, if we express the level of collimation line above datum by Height of Instrument (HI), then RL of Q = HI + V − h In case of a depressed sight RL of Q = HI – V − h ( Staff Normal ) Tacheometric Formula

21. Field method for determining constant of Thacheometer Fix instrument at O Take stadia hair readings at A,Band C . Measure distances OA,OB,OC with help of tape. Calculate the staff interval at A,B and C . Put values of horizontal distances and staff intervals in tacheometric formula and make three equations. Solving these equations constants (f/ i ) and ( f+c ) can be obtained.

22. Limitations of Tacheometric Surveying Less accurate method and chaining is completely eliminated. Small error in observing stadia rod gives large error in calculation of horizontal distance and RL. Therefore method is used for relatively small precision job. It has been recommended that error in single horizontal distance should be in 1 in 500. This method is not suitable for precise survey.

23. Errors in Tacheometric Surveying