For measurement of vertical angles through theodolite, the instrument should be levelled with reference to the ________ bubble and the ________ clamp that is used to rotate the telescope in the vertical plane.
altitude; vertical circle
Understanding Vertical Angle Measurement with a Theodolite
When measuring vertical angles using a theodolite, precise setup and operation are crucial. A vertical angle is the angle between a line of sight to a point and the horizontal plane. To ensure accurate measurement of these angles, the instrument must be properly oriented and leveled.
Levelling for Vertical Angle Measurement
For measuring vertical angles, the instrument needs to be accurately leveled with respect to the vertical axis. The bubble tube that is specifically designed for this purpose and directly influences the reading on the vertical circle is the altitude bubble (also sometimes called the vertical circle bubble or plate bubble). Centering this bubble ensures the vertical circle is truly vertical, which is essential for measuring angles relative to the horizontal plane.
Mechanism for Vertical Rotation
The telescope of a theodolite rotates in a vertical plane to sight points at different elevations. This rotation is controlled by the vertical circle assembly. This assembly includes a graduated vertical circle, a vertical clamp, and a vertical tangent screw. The vertical clamp allows the telescope to be fixed at a specific vertical angle, while the vertical tangent screw permits fine adjustment of the vertical pointing.
The question refers to the clamp used to rotate the telescope in the vertical plane. This is the vertical clamp associated with the vertical circle.
Connecting Components to the Question
The question asks for the specific bubble used for levelling when measuring vertical angles and the specific clamp used for rotating the telescope in the vertical plane.
- The bubble used for accurate levelling for vertical angles is the altitude bubble.
- The clamp used to fix the telescope's vertical position, allowing controlled rotation in the vertical plane, is part of the vertical circle assembly (specifically the vertical clamp).
Therefore, the instrument should be levelled with reference to the altitude bubble and the clamp associated with the vertical circle is used to rotate the telescope in the vertical plane.
Summary of Theodolite Components for Vertical Angles
| Component | Function | Relevance to Vertical Angles |
|---|---|---|
| Altitude Bubble | Ensures vertical axis is truly vertical. | Critical for accurate vertical angle measurement relative to the horizontal. |
| Vertical Circle | Graduated circle for reading vertical angles. | Provides the scale for measuring the angle. |
| Vertical Clamp | Locks the telescope at a specific vertical angle. | Allows fixed aiming or controlled adjustment. |
| Vertical Tangent Screw | Fine adjustment for vertical aiming. | Used for precise pointing after clamping. |
Revision Table: Key Theodolite Components for Angle Measurement
| Component | Primary Purpose | Relevant Angle Type |
|---|---|---|
| Tribrach and Foot Screws | Initial coarse levelling and centering over a point. | Both Horizontal and Vertical |
| Plate Bubbles (or Circular Bubble) | Coarse levelling of the horizontal plate. | Both Horizontal and Vertical (initial) |
| Altitude Bubble | Fine levelling for vertical angle measurement. | Vertical |
| Horizontal Circle | Measures angles in the horizontal plane. | Horizontal |
| Upper Clamp / Upper Tangent Screw | Locks/fine adjusts movement relative to the graduated horizontal circle. | Horizontal |
| Lower Clamp / Lower Tangent Screw | Locks/fine adjusts movement of the entire instrument relative to the lower plate (used for orienting). | Horizontal (for orientation/repetition) |
| Vertical Circle | Measures angles in the vertical plane. | Vertical |
| Vertical Clamp / Vertical Tangent Screw | Locks/fine adjusts vertical movement of the telescope. | Vertical |
Additional Information: Types of Angles Measured by Theodolite
A theodolite is a versatile surveying instrument primarily used for measuring angles. The two main types of angles measured are:
- Horizontal Angles: These are angles measured in the horizontal plane between two points from the instrument station. They are used for determining directions and laying out points. Measured using the horizontal circle.
- Vertical Angles: These are angles measured in the vertical plane between a line of sight and the horizontal plane. They are used for determining elevations or heights. Measured using the vertical circle. Vertical angles can be measured as angles of elevation (above horizontal) or angles of depression (below horizontal). Some instruments measure zenith angles (angle from the upward vertical) or nadir angles (angle from the downward vertical).
Important Questions from Compass Surveying and Theodolite
Which of the following statement is CORRECT for whole circle bearing?
Least count of a theodolite is ________
The meridians that are parallel to the directions taken by freely moving magnetized needles, as in a compass are called as:
The following statements given pertain to theodolite used in surveying. Identify the incorrect statement
The use of Electronic Digital Theodolite attached to a total station is to measure:
