Tacheometry 2.pptx

Tacheometry 2.pptx

• 1.
  Field-Work in TacheometricSurvey
• 2.
  1. Suitability 2. Equipment 3.Field-Party 4. Procedure 5. Field-Book
• 3.
  Tacheometric survey ismainly suitable for contouring because simultaneous calculations for horizontal distances and differences of level are possible from the same set of the observed values. It is particularly useful in hilly areas where the slopes are steep and country is rough and therefore ordinary levelling and chaining become rather difficult. It is also suitable for carrying out traverses and filling in detail in rough and rugged terrain where measurement of distances by chain is not easy.
• 4.
  (i) A tacheometer, (ii)A stadia rod or levelling staff, (iii) A tape, (iv) Ranging rods, (v) Pegs etc.
• 5.
  (i) The surveyorwho is in-charge of the party and supervises all operations such as reconnaissance, selection and location of stations, positions of staff men, etc. (ii) The observer who is responsible for actual observations. (iii) The recorder who records the observations in the field- book and assists the observer. (iv) Three or more staff men. (v) Two or more axe-men for clearing and fixing pegs etc.
• 6.
  The tacheometric surveyshould be conducted in the following steps: (i) Set up the instrument over the station-point. Centre and level it accurately. (ii) Set up vertical vernior to zero and measure the height of the instrument i.e. height from top of peg to centre of the object-glass with a tape or stadia rod through the object-glass. (iii) Orient the instrument correctly at the first station of the traverse either with reference to the true meridian or with reference to the magnetic meridian. (iv) Sight the staff held on the nearby bench mark and observes the vertical angle, the bearing and the readings of the three hairs. If there is no bench mark nearby, flying levelling may be done from any B.M. to establish another one near the area of the survey.
• 7.
  (v) Sight allthe representative points around the station and within the range of the instrument and observes at each the bearing, the vertical angle and the staff readings at the three wires, the bearing being taken to the nearest 5′ and the vertical angle to the nearest 1′. (vi) Take a fore sight on the second traverse-station and observe the bearing, the vertical angle and the staff readings of three wires. (vii) Shift the instrument and set it up at the second station. (viii) Measure the height of instrument as before. (ix) Back sight the staff held on the first station and observes the bearing, the vertical angle and the staff readings of three wires. (x) Sight all the points around the second station and within the range of the instrument as described above. (xi) Take a fore sight on the third station and take necessary observations. (xii) Proceed similarly at each of the successive points.
• 9.
  Errors in StadiaSurveying: The sources of errors in stadia measurements are as follows: 1. Instrumental Errors. 2. Personal Errors. 3. Natural Errors.
• 10.
  1. Instrumental Errors: (i)Imperfect adjustment of the tacheometer: This error can be eliminated by carefully adjusting the instrument, particularly the altitude bubble. (ii) Incorrect divisions on the stadia rod: In ordinary work, this error is negligible but for precise work, the error can be minimised by using the standardised rod and applying corrections for incorrect length to the observed stadia intervals. (iii) Incorrect value of the multiplying constant (f/t): This is the most serious source of error. The value of the multiplying constant should be tested before commencing the work by comparing the stadia distances with measured distances during the hours which correspond to those of field-observations.
• 11.
  2. Personal Errors: (i)Inaccurate centering and levelling of the instrument. (ii) Non-vertical by of the staff or rod. It may be eliminated by using a plumb-line or a small circular spirit level with the staff. (iii) Inaccurate Focussing. (iv) Reading with wrong hair. The personal errors can be eliminated by applying habitual checks.
• 12.
  3. Natural Errors: (i)High wind: The work should be suspended in high wind. (ii) Unequal refractions: This error is prominent during bright sunshine and mid-day hours of hot summer days. The work can be suspended under such circumstances. (iii) Unequal expansion: The instrument should be protected by an umbrella during hot sun. (iv) Bad visibility: It is caused by glaring of strong light coming from the wrong direction.