CiIvil Surveying - Theodolite Surveying
1.Define: Theodolite surveying. What are the uses of a theodolite?
Theodolite surveying is a branch of surveying, in which the theodolite is accurate instrument used for the measurement of horizontal angles, vertical angles. It can also be used for various purposes such as laying off horizontal angles, locating points on a line, prolonging survey lines, establishing grades, determining differences in elevation, etc,.
2. Why a type of theodolite is called a transit theodolite?
A transit theodolite is one is which the line of sight can be reversed by revolving the telescope through 1800 in the vertical plane.
3. List the essential parts of a theodolite.
The telescope, The vertical circle, The index frame (T- frame),
The levelling head, The scale plate (Lower), The Vernier plate (Upper). The tripod, Plumb bob, The compass, Focusing screws,
The upper and lower clamp and its tangential screws, Vertical clamp screw, Foot screws. etc,.
4. Differentiate between the Vernier theodolite and Micrometer theodolite.
In Vernier theodolite the verniers are fitted to read the angles upto 20”. In Micrometer theodolite micrometers are fitted to read the angles.
5. List the essential qualities of a theodolite telescope. The essential parts of the telescope:
(i).Objective (ii).Eye-piece, (iii). Diaphragm, (iv). Body and focusing device.
The essential qualities of a theodolite telescope:
(i). The real image must be formed in front of the eye-piece.
(ii). The plane of the image must coincide with that of the cross-hairs.
6. What are the temporary adjustments of the theodolite?
1. Setting up.
2. Levelling up.
3. Elimination of parallax.
(i). Focusing the eye piece. (ii). Focusing the Objective.
7.List out the permanent adjustments of Theodolite.
1. Adjustment of plate level (Plate level test).
2. Adjustment of line of sight ( Cross- hair test).
3. Adjustment of horizontal axis.(Spire test).
4. Adjustment of vertical index frame and altitude bubble.(Vertical arc test).
8. List out the fundamental lines of Theodolite
1. The vertical axis
2. The horizontal axis or trunnion axis
3. The line of collimation or line of sight.
4.Axis of plate level.
5. Axis of altitude level.
9. What is an anallatic lens? What is the use of an anallatic lens?
It is a special convex lens fitted between the object glass and eye piece, at a fixed distance from the object glass, inside the telescope of a tacheometer.
The use of an anallatic lens is to reduce the additive constant ( C ) to zero. Tacheometric equation, D = Ks + C. Where,
D = horizontal distance between the staff and the instrument station. S = staff intercept
K = multiplying constant. C = additive constant.
10. Name the two methods of measuring horizontal angles using a thedolite. When each method is advantageously used?
The two methods: (i). Repetition method.
(ii). Reiteration method (Direct method).
The method of repetition is preferred for the measurement of a single angle. The method of reiteration is preferred in triangulation, where a number of
angles may be required at one point by the instrument.
The method of repetition appears to be better, it is more time consuming and even many repetitions may yield ordinary results.
11. State what errors are eliminated by repetition method.
1. Errors due to imperfect eccentricity of verniers and centres are eliminated
by reading both verniers and taking the mean of the reading.
2. Errors due to imperfect adjustment of the instrument are eliminated by taking face left and right observations.
3. Errors due to inaccurate graduations on the main scale are eliminated as the readings are spread over different parts of the circle.
4. Errors due to inaccurate bisection of the signal are eliminated, as they tend to balance each other. All the errors are also minimized as the number of repetitions divides the sum.
12. What are the methods used to plot the traverse?
1. By parallel Meridians Through Each Station.
2. By Included Angles
3. By Paper Protractor.
4. By rectangular co-ordinates.
5. Plotting by tangents and chords.
13.Define closing error.
If a closed traverse is plotted according to the field measurements, the end point of the traverse will not coincide exactly with the starting point, due to the errors in the field observations. Such as error is known as closing error.
Closing error e = ? ( ?L) 2 + ( ?D) 2.
where, ?L = sum of latitudes.
?D = sum of departures.
The direction of closing error
tan ? = ?D / ?L.
14. Define: Balancing. What are the methods used to balancing the traverse?
The term balancing is generally applied to the operation of applying corrections to latitudes and departures. So that ?L = sum of latitudes = 0; ?D = sum of departures = 0. This is applied only for closed traverse
Methods used to balancing (adjusting) the traverse:
1. Bowditch’s rule
2. Transit rule.
3. Graphical method.
4. Axis method.
15.Explain the Bowditch’s rule in balancing the traverse.
The Bowditch’s rule (compass rule) is used to balance a traverse where the linear and angular measurements are of equal precision. In this method the error in linear measurements are proportional to ?l and the error in angular measurements are inversely proportional to ?l ,where l is the length of the line.
Correction to latitude (or departure) of any side
= Total error in latitude (or departure) x [ Length of that side / Perimeter of traverse.]
C L = ? L .( l / ?l)
C D = ? D .( l / ?l)
? L = Total error in Latitude.
? D = Total error in Departure.
l = Length of the side ?l = Perimeter of traverse.
16. What is Gale’s table? What is its use?
Traverse computations are usually done in a tabular form. One such form is known as Gale’s Table.
It is used to balance the traverse and also used to find the area of the closed
17. Define: Omitted measurements.
Sometimes it is not possible to take all the measurements due to obstacles. Such missing quantities are known as omitted measurements. They can be calculated from the known latitudes and departures.
18. What is closed traverse? What are the two checks applicable in this case?
When the lines form a circuit, which ends at the starting point, it is known as closed traverse.
(i). Sum of interior angles = (2n – 4) 90 (ii). Sum of exterior angles = (2n +4) 90
Where, n= number of sides of the traverse.
19. Define: Trigonometrical leveling or Heights and Distances.
Trigonometrical leveling is an indirect method of levelling. The relative elevations of various points are determined from the observed vertical angles and horizontal distances by the use of trigonometrical relations. The vertical angles are measured with a theodolite and the horizontal distances are measured with a tape or chain. This method is also known as Heights and Distances.
20. What are the methods used to find the elevations of the points in the case of inaccessible points? Differentiate that?
1. Single plane method
2. Double plane method.
Single plane method
Two instrument stations are chosen in line with the object.
The two vertical angles are measured in the same vertical plane.
Horizontal angles are not required.
Double plane method
Two instrument stations are chosen which are not in line with the object.
The two vertical angles are measured in two different vertical planes.
Horizontal angles are also measured.