Understanding Theodolite Permanent Adjustments

Theodolites are precise surveying instruments used for measuring horizontal and vertical angles. To ensure accurate measurements, the different parts of the theodolite must be in their correct relative positions. Permanent adjustments are procedures performed to achieve these correct relationships and eliminate errors caused by faulty construction or wear and tear. These adjustments are called 'permanent' because they aim to correct internal relationships within the instrument itself.

It is crucial to perform these adjustments in a specific sequence, as adjusting one part might affect the correct positioning of another. Performing them out of order can lead to needing to repeat adjustments multiple times.

Order of Permanent Adjustments

The question asks for the correct order of performing the permanent adjustments listed. Based on the provided options and the correct answer, the sequence is 1-2-5-4-3-6. Let's break down what each numbered step represents:

1. Plate level test
2. Cross- hair ring test
3. Bubble tube adjustment test
4. Spire test
5. Collimation in azimuth test
6. Vertical circle test

So, the sequence 1-2-5-4-3-6 corresponds to performing the adjustments in the following order:

1. Plate level test
2. Cross- hair ring test
3. Collimation in azimuth test
4. Spire test
5. Bubble tube adjustment test
6. Vertical circle test

Detailed Explanation of Each Adjustment in Sequence 1-2-5-4-3-6

1. Plate Level Test

Condition: The axis of the plate level should be perpendicular to the vertical axis of the instrument.

Purpose: To ensure the instrument can be properly levelled so that the horizontal circle lies in a horizontal plane.

Adjustment: The instrument is levelled roughly. The plate bubble is brought parallel to two foot screws and centred. Then it's rotated 90 degrees and centred using the third foot screw. This is repeated until the bubble remains central in any position. If the bubble doesn't remain central, the plate level bubble tube's capstan headed nuts are adjusted until it does.

2. Cross-hair Ring Test (Horizontal Cross-hair Test)

Condition: The horizontal cross-hair should lie in a plane perpendicular to the vertical axis.

Purpose: To ensure that when the instrument is levelled, the horizontal cross-hair defines a truly horizontal line of sight.

Adjustment: Level the instrument. Sight a well-defined point A using the horizontal cross-hair. Rotate the instrument slightly using the tangent screw so the point moves towards one end of the cross-hair. If the point appears to move along the horizontal cross-hair, the condition is satisfied. If not, the diaphragm (cross-hair ring) is rotated by loosening screws holding it and adjusting until the point travels along the cross-hair.

3. Collimation in Azimuth Test (Line of Sight Adjustment)

Condition: The line of sight (defined by the intersection of the cross-hairs and the optical center of the objective lens) must be perpendicular to the horizontal axis (trunnion axis).

Purpose: To ensure that when the telescope is rotated horizontally, the line of sight sweeps out a truly vertical plane.

Adjustment: Set up and level the instrument. Sight a point A far away. Transit the telescope (plunge it). If the line of sight is correct, the point A' (the same point A after transiting) should lie on the line of sight in the new position. If not, a new point B is marked on the line of sight. The distance A'B is bisected at C. The vertical cross-hair is then adjusted using the horizontal capstan screws on the diaphragm until the line of sight passes through C. This adjustment ensures the line of sight is perpendicular to the horizontal axis.

4. Spire Test (Horizontal Axis Test / Trunnion Axis Test)

Condition: The horizontal axis (trunnion axis) must be perpendicular to the vertical axis.

Purpose: To ensure that when the telescope is elevated or depressed, the line of sight remains in a single vertical plane.

Adjustment: Set up and level the instrument near a tall building or structure. Sight a high point A on the structure with the telescope elevated. Depress the telescope and mark a point B exactly below A on the ground or a lower part of the structure. Transit the telescope. Sight point B again. Elevate the telescope. If the line of sight is correct, it should pass through the original point A. If it passes through A', the distance AA' is bisected at C. The trunnion axis (spire) is adjusted by raising or lowering one end of the axis using adjusting screws located on the standards, until the line of sight passes through C when pointing at A. This makes the horizontal axis perpendicular to the vertical axis.

5. Bubble Tube Adjustment Test (Altitude Level Adjustment)

Condition: The axis of the altitude level (bubble tube attached to the telescope) must be parallel to the line of sight when the vertical circle reading is zero.

Purpose: To ensure that when the altitude bubble is centered, the line of sight is truly horizontal, allowing accurate measurement of vertical angles and use as a level.

Adjustment: Set up and level the instrument. Use a peg test or reciprocal levelling method to determine the true horizontal line between two points. Set the line of sight along this true horizontal. The vertical circle reading should be 0°00'00". If not, adjust the vertical circle index. Once the vertical circle reads zero for a horizontal line of sight, the altitude bubble should be central. If it is not, adjust the bubble tube's capstan screws until the bubble is centered without disturbing the telescope's elevation.

6. Vertical Circle Test (Vertical Circle Index Adjustment)

Condition: The vertical circle must read zero when the line of sight is horizontal and the altitude bubble is centered.

Purpose: To ensure accurate vertical angle measurements.

Adjustment: Level the instrument and centre the altitude bubble. Sight a point at a horizontal angle (level). The vertical circle should read 0°00'00". If not, read the vertical angle (index error). The index mark or the vernier/scale is adjusted using its adjusting screws so that it reads zero when the telescope is horizontal and the altitude bubble is centered. Alternatively, the index error can be noted and applied as a correction to all vertical angle readings.

Following the sequence 1-2-5-4-3-6 addresses the primary sources of instrumental error in a systematic way.

Revision Table: Key Theodolite Adjustments

Additional Information: Temporary vs. Permanent Adjustments

Theodolite adjustments are broadly classified into two types:

• Temporary Adjustments: These are performed at every station before starting observations. They ensure the instrument is ready for use at that specific location. The typical order is setting up the tripod, centering over the station mark, levelling the instrument (using the plate levels and foot screws), and focusing the telescope (on the object and the cross-hairs).
• Permanent Adjustments: These are performed periodically or when the instrument has been subjected to rough handling or transport. They correct the relative positions of the instrument's components to eliminate fundamental errors. The order is specific and crucial, as detailed in the solution above.

Permanent adjustments are essential for the long-term accuracy and reliability of the theodolite's measurements.