Theodolite surveying

THEODOLITE SURVEYING

For measuring angles in survey work, the instruments used are: Compass and  Theodolite.

Compass Vs Theodolite: The compass does not measure the angle between two lines directly, but measures the angle between the magnetic meridian and the line. But, the theodolite measures (i) the angle between two lines directly and (ii) the bearing of the lines.

Uses: Theodolite is a precise instrument used for measuring horizontal and vertical angles directly in surveying, including determining differences in elevation, extending survey lines, locating points on a survey line, etc.

Types of Theodolite: Theodolite is classified as Transit Theodolite and Non-transit Theodolite. In the transit theodolite, the line of sight can be reversed / inverted (i.e., transited) by revolving the telescope through 180° about a horizontal axis in the vertical plane. It is most widely used nowadays. In the non-transit theodolite, the telescope cannot be transited. It is obsolete.

Fig. 34 shows the pictorial view of Transit Theodolite. It is mounted on a Tripod Stand. The tripod stand consists of three solid or framed legs.

 

1. TRANSIT THEODOLITE

Description (Fig. 35)

1. Telescope

The telescope is mounted on a horizontal spindle, known as Horizontal Horizontal Axis. The Horizontal Axis is supported on the Standards. The telescope Telescope is rotated in horizontal or vertical planes to sight the distant objects.

2. Vertical Circle

It is connected to the horizontal axis. Hence, the vertical circle rotates when the telescope is rotated about the horizontal axis. It is graduated from 0° to 360° clockwise.

Vertical angles are measured by Verniers, fixed diametrically opposite to each other. Using a Vertical Circle Clamping Screw, the telescope can be set in any desired position in the vertical plane.

3. Altitude Bubble: For measuring vertical angles, the transit theodolite is levelled with reference to the Altitude Bubble.

4. Upper Plate or Vernier Plate: It is attached to the Inner Solid Spindle. It carries two verniers A and B, with magnifiers to magnify the readings. The magnifiers are placed 180° apart for reading horizontal angles. The upper plate supports the Standard at its upper surface. It is connected to the Lower Plate.

5. Lower Plate or Scale Plate: It is a graduated horizontal circular plate attached to the Outer Hollow Spindle. It is graduated from 0° to 360° in clockwise direction. When the upper plate is clamped using Upper Plate Clamping Screw, both the upper and lower plates move together as one unit. When upper plate is loosened, it rotates with respect to the lower plate and the vernier readings change.

6. Plate Levels or Plate Bubbles: Two spirit levels called Plate Levels or Plate Bubbles are used to level the instrument. They are fixed at 90° to each other on the upper vernier plates.

7. Plumb Bob: A Plumb Bob is suspended from the hook fitted to the bottom of the vertical axis. Its purpose is to center the theodolite exactly over the station mark.

 

2. TERMINOLOGY

1. Centering: It is the process of setting up the theodolite exactly over a station mark. Centering may be done by plumb bob.

2. Levelling: It is the process of making vertical axis of the instrument truly vertical.

3. Axis of Telescope: It is an imaginary line joining the optical center of the object glass and the center of the eye piece.

4. Horizontal Axis: It is an axis about which the telescope is rotated in a vertical plane. It is also known as Trunnion Axis, Elevation Axis or Transverse Axis.

5. Vertical Axis: It is an axis about which the instrument is rotated in a horizontal plane.

6. Transiting: Transiting is the process of rotating the telescope in the vertical plane through 180° about the horizontal axis.

7. Axis of Plate Level: It is a straight line tangential to the longitudinal curve of the plate level tube at its center. It is horizontal when the bubble is centered.

8. Axis of Altitude Level: It is the straight line tangential to the longitudinal curve of the altitude level tube at its center. It is horizontal when the bubble is centered.

9. Face Left Observation: It is an observation of horizontal or vertical angle when the face of the vertical circle is to the left of the observer.

10. Face Right Observation: It is an observation of horizontal or vertical angle when the face of the vertical circle is to the right of the observer.

11. Changing the Face: It is an operation of bringing the face of the telescope from face left condition to face right condition or vice versa. It is achieved by transiting the telescope and then swinging it through 180°.

12. Line of Sight or Line of Collimation: It is an imaginary line passing through the intersection of horizontal and vertical cross-hairs of the diaphragm and the optical center of the object glass.

13. Swinging the Telescope: It is the process of rotating the telescope in a horizontal plane.

 

1.  Fundamental Lines and Their Relations

Following are the fundamental lines of a transit:

1. Vertical Axis

2. Horizontal Axis

3. Line of Collimation

4. Axis of Plate Level

5. Axis of Altitude Level

Desired Relations: A theodolite in proper adjustment must have the following relations between fundamental lines:

1. Axis of Plate Level must be perpendicular to the Vertical Axis.

2. Horizontal Axis must be perpendicular to the Vertical Axis.

3. Axis of Altitude Level must be parallel to the Line of Collimation.

4. Line of Collimation must be perpendicular to the Horizontal Axis.

5. Vertical Circle Vernier must read zero, when the Line of Collimation is horizontal.

 

3.  DIRECT METHOD OF MEASURING HORIZONTAL ∠ POR

A horizontal angle is the angle formed by two intersecting vertical planes at their point of intersection. It is measured in the horizontal plane using the Method of Repetition.

1. See Fig. 36. To measure the horizontal ∠ PQR between two stations P and R at the instrument station Q, pegs or ranging rods are erected at P and R.

2. Theodolite is set at Q, centered and levelled with reference to the plate levels.

3. Turn the upper and lower plates in the opposite direction. Set the vernier A to 0 using the upper clamp.

4. Then clamp both the plates together.

5. Note the readings of verniers A and B.

6. Turn the telescope towards the left signal at P. Bisect P using lower tangent screw.

7. Then turn the telescope clockwise to bisect signal at R. Bisect R by upper tangent screw.

8. Read both verniers to get the approximate value of the angle PQR with one face.

9. Repeat the above by changing the face of the instrument to right. The mean of the two vernier readings with different faces gives the average horizontal angle.

 

4. DIRECT METHOD OF MEASURING VERTICAL ∠POH

Vertical angle is the angle made by the inclined line of sight with the horizontal plane through the horizontal axis.

1. See Fig. 37. P is the object at station Q. Level the instrument with reference to the plate level.

2. Turn the instrument about vertical axis until the altitude bubble is parallel to the line joining any two foot screws below the lower plate.

3. Turn the telescope through 90° so that the altitude bubble is parallel to the line joining the third foot screw and the midpoint of the line joining the two foot screws.

4. Bring the bubble to its center by turning the third foot screw.

5. Turn back the telescope through 90° so that it is again parallel to the two foot screws.

6. Then, direct the telescope towards P. Bisect P using the vertical tangent screw.

7. Read both the verniers of the vertical circle. Mean of the two verniers give the vertical angle РОН.