THEODOLITE SURVEYING LESSON 5

THEODOLITE LESSON  5

HORIZONTAL ANGLE
         When two lines are meeting on a horizontal plane the angular distance between them is called “horizontal angle”
       These angles are measured with help of horizontal (plate) circle and corresponding verniers “A” and “B”
                                
MEASUREMENT OF HORIZONTAL ANGLE
         By the method of repetition
         By the method of reiteration

REPETITION METHOD
         This method is adopted for precise works
         Face left and face right observations are essential
         All the readings should be entered in field book

STEPS INVOLVED IN MEASURING HORIZONTAL ANGLES
                Let   “O” is an instrument station
                        “P” and “Q” are objects
                        ( see Figure- 2 (a) and (b) 

         Set up theodolite over station “O” and complete the temporary adjustments accurately
         Bring the vernier “A” to zero degrees
         Release lower clamp screw and sight to object “P” and clamp it firmly
         Bisect the object “P” accurately with help of  corresponding slow motion screw
         Release the upper clamp screw and sight to object  “Q” and clamp it
         Bisect the object “Q” accurately with help of  corresponding slow motion screw
         Read main scales and corresponding vernier scales  “A” and “B”
         Enter the readings in the proforma shown in table -1 


            Take average of two readings. This reading is the required horizontal angle “POQ”
             Change the face of the instrument, repeat the process  as explained above
             Take average of two readings. This reading is the required horizontal angle “POQ”
             Take average of face left and face right to get further  accurate horizontal angle   “POQ”

METHOD OF REPETITION
         Measure the horizontal angle for ‘n’ no of times so that the included angle accumulates to approximately ‘n’ times the actual horizontal angle.
          Divide the total reading by ‘n’ to get accurate horizontal angle between P and Q.



The following errors are eliminated in this method
          The errors due to the eccentricity of the centers and of  the verniers are eliminated by taking average of both  verniers
          Imperfect adjustment of the line of collimation and the  horizontal axis of the telescope are eliminated by face  left and face right observations
          The errors of graduations are minimized by measuring the angle on different parts of the circle
         The errors in the pointings tend to compensate each other and the remaining error is minimized by the division
NOTE:    Error due to improper levelling of the bubble cannot be eliminated. However this can be eliminated by perfect levelling of the instrument


REITERATION METHOD
             This method is used when several angles are to be measured at a point
               This method consists of measuring several angles  successively and finally closing  (ending) on first  station
       Let 
             A,B,C,D are objects
            “O” is an instrument  station


         Set up theodolite over station “O” and complete the temporary adjustments accurately
         Bring index of verniers  “A”   to zero degrees.
         Release lower clamp screw and sight to object “A” and clamp it firmly.
         Bisect the object “A” accurately with the help of corresponding slow motion screw
         Release the upper clamp screw and sight to object “B” and clamp it  
         Bisect the object “B” accurately with help of  corresponding slow motion screw
         Read main scales and corresponding vernier scales “A”  and “B”
         Enter the readings in the proforma shown in table -1  
         Release the upper clamp screw and sight to object “C” and clamp it
         Bisect the object “C” accurately with help of  corresponding slow motion screw
         Read main scales and corresponding vernier scales “A” and “B”
          Enter the readings in the profarma shown in table -1  
         Repeat the process until all objects are completed
         Enter the respective readings in the profarma shown in  table -1
         Change the face of the instrument and repeat the process    as  explained above
         Take average of face left and face right to get further  accurate respective angle

The following errors are eliminated in this method
              Graduation errors are eliminated
              The errors due to the eccentricity of the centers and of the verniers are eliminated   taking average of both verniers
              Imperfect adjustment of the line of collimation and the horizontal axis of the telescope are eliminated by face  left and  face right observations



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