CoursesGeological Survey

Determine Horizontal angles of Survey Scheme Using Theodolite

Objective – Survey Scheme

The main objective of the survey scheme is to measure the horizontal angles of the scheme using the theodolite so that these values can be used for the traverse computations of the scheme.

Apparatus

  • Theodolite
  • Tripod Stand
  • Ranging Rod

Scheme

The scheme of which we are going to measure the horizontal angles is following:

Example image derived from location: UET Lahore, Pakistan

Methods of Measuring Horizontal Angles

There are two methods:

  • Repetition
  • Reiteration

Repetition Method

A single angle <AOB is measured usually four times and then its average value is taken.

Procedure

  1. To measure the angle <AOB , the theodolite is set at point O and then repetition mode of the theodolite is turned on and the Face of the theodolite is taken as left.
  2. Bisect the ranging rod placed at the station point A and set the Theodolite angle as (zero).
  3.  Now turn the theodolite and bisect the ranging rod at station point B. Press the Hold button (a reading will be shown on the display screen).
  4. Now again bisect the station Point A and Press R\L button.
  5. Bisect Point B again, and Press Hold button.
  6. Repeat this process 4 times and take the average value of angle by dividing the Accumulated angle by number of readings taken.
equation

Now repeat this process by taking the Theodolite as Face Right.

Face Left readings and Face Right Readings are taken to ensure the Correctness of the Angle, thus measured.

Re-iteration Method

This method is used when two or more angles are to be measured at one Station point.

Procedure

  1. Set the theodolite at station Point O.
  2. Bisect the point A measure the <AOB by turning the theodolite and bisecting the point B.
  3. Now Measure the <BOC by bisecting the point C
  4. The Horizon  is closed by measuring the external angle <COA
  5. The sum of all angles must be equal to 36.

Comparison

Repetition MethodRe-iteration Method
1. When only one angle is to be measured at one station point

2. It is comparatively more accurate.

3. Horizon is not closed at the instrument station.

4. When the precision of measurement of a horizontal angle is desired to be more than the least count of the instrument, this method is used.

5. In this method, an angle is measured four times and the average is taken.
1. When two or more angles are to be measured at one station point.

2. Horizon is closed at the instrument station and sum of all angles must be equal to 360o .

3. In this method, sub-angles at one station point are measured.
The sum of angles after closing the horizon should be equal to 360o .
Comparison of Repetition Method and Re-iteration Method

Scheme

The horizontal angles of the above-mentioned scheme are measured by using the Re-Iteration method.

Observation and Calculations

The angle measurements for different angles at different stations by reiteration method are following:

I.SAngle ObservedFaceObserved ValueMean ValueErrorCorrected Mean ValueRemarks
BABCL3403’50”3403’50”+05”3403’45”—-
R3403’50”
CBDL5609’25”5609’15”5609’15”
R5609’05”
DBAL26947’00”26947’00”26947’00”
R26947’00”
ACADL6030’35”6030’37.5”-15”6030’42.5”—-
R6030’40”
DABL3931’30”3931’35”3931’40”
R3931’40”
BACL25957’30”25957’32.5”25957’37.5”
R25957’35”
GDGEL1826’00”1825’55”-2.5”1825’56.5”—-
R1825’50”
FGEL4704’50”4704’50”4704’50”
R4704’55”
EGFL29429’10”29429’12.5”29429’14”
R29429’15”
CACBL4552’40”4552’40”-25”4552’50”—-
R4552’40”
BCDL3705’30”3705’25”3705’30”
R3705’20”
DCAL27701’30”27701’30”27701’40”
R27701’30”
FGFEL1419’35”1419’32.5”+25”1419’22.5”—-
R1419’30”
EFGL34540’50”34540’52.5”34540’37.5”
R34540’55”
          E  DEF      L  8559’10’’      8559’05’’            +10’’    8559’02’’                   —–
R8559’10’’
  FEGL  11840’0’’  11840’0’’  11839’56’’
R118⁰40’0’’
GEDL15521’10’’15521’05’’15521’02’’
R15521’00’’
DBDAL5012’20”5012’50”00”5012’50”—-
R5013’40”
ADCL3631’20”3631’17.5”3631’17.5”
R3631’15”
CDEL9242’25”9241’17.5”9241’17.5”
R9242’10”
EDBL18034’30”18034’35”18034’35”
R18034’40”

Learning Outcomes

1. What is a transit theodolite?

Ans: The theodolite in which telescope can be rotated about its horizontal axis in the vertical plane through 180o is called as the transit theodolite. 

2. What are fundamental lines of theodolite?

Ans: Vertical axis, Horizontal axis, axis of the bubble tube, Line of collimation.

3. What are temporary adjustments of a theodolite?

Ans: Fixing ,Centering, Leveling and focusing.

4. What are the different methods of finding the horizontal angles?

Ans:    1- Repetition

            2- Reiteration

5. Define repetition method?

Ans: When only one angle is measured from a point four times then this method is called repetition method.

6. In which condition is reiteration method used?

Ans: This method is used if there is more than one an

7. What is meant by face left and face right observations?

Ans: When the vertical circle is on the left side of the observer then it is called face left obsevation and when the vertical circle is at right side of the observer then this position is called face right position

8. What is another name for circular bubble?

Ans: Bull’s eye bubble , pill box bubble.

9. Which arithmetic check is applied for reiteration method?

Ans: The sum of all angles measured at a point should be equal to 360o .

10. What does closed horizon means in reiteration method?

Ans: It means after measuring all the angles at the particular station the sum of all these angles should be equal to 360o .

Shoaib Anjum

Shoaib Anjum is a Geological Engineer. However, his interests extend towards Writing, Designing, Web Development, SEO, Blogging etc., Shoaib has assisted a great number of people in launching their sites/services over a course of 10+ years and proven to provide results that were much more than satisfactory. Shoaib's working methods guarantee competitive results at the initial stages of his projects.

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