Two Peg Test: Correcting Line of Collimation in Levels

In surveying, permanent adjustment of a level instrument refers to a series of operations performed to establish and maintain the fundamental relationships between the various parts of the instrument. These adjustments ensure that the instrument operates accurately and consistently, providing reliable measurements. One crucial permanent adjustment involves the line of collimation.

Line of Collimation and its Importance

The line of collimation, also known as the line of sight, is an imaginary line passing through the optical center of the objective lens and the intersection of the crosshairs (vertical and horizontal) in a surveying level. For accurate leveling, this line must be truly horizontal when the instrument's bubble is centered, meaning it should be parallel to the axis of the level tube.

If the line of collimation is not truly horizontal (i.e., it is inclined), all readings taken with the level will be consistently too high or too low, leading to errors in determining elevation differences. The two peg test is specifically designed to check and correct this crucial condition.

Purpose of the Two Peg Test

The primary purpose of the two peg test is to ensure that the line of collimation of the level is parallel to the axis of the level tube, or equivalently, that the line of collimation is truly horizontal when the bubble is centered. This test helps in detecting and eliminating any collimation error, which is the angular deviation of the line of sight from the true horizontal plane.

Procedure and Correction through Two Peg Test

The two peg test involves setting up two points, A and B, at a considerable distance apart (e.g., 60-100 meters) and then taking readings with the level instrument placed at two different positions:

1. First Setup (Midway): The level is set up exactly midway between points A and B. Since the instrument is equidistant from both points, any error in the line of collimation will affect both readings equally, thus cancelling out when calculating the difference in elevation. The true difference in elevation between A and B is obtained by subtracting the staff reading at B from the staff reading at A.
   • Let readings be $R_A_1$ (at A) and $R_B_1$ (at B).
   • True difference in elevation, $D_{True} = R_A_1 - R_B_1$.
2. Second Setup (Near one point): The level is then moved to a point very close to either A or B (e.g., 5 meters from A and 65 meters from B). From this position, readings are again taken on staff held at A and B. Because the distances are now unequal, any collimation error will have a differential effect on the readings.
   • Let readings be $R_A_2$ (at A) and $R_B_2$ (at B).
   • The observed difference in elevation, $D_{Observed} = R_A_2 - R_B_2$.
3. Calculating Collimation Error: If the instrument is in perfect adjustment, $D_{True}$ should be equal to $D_{Observed}$. If they are not equal, it indicates a collimation error. The error is calculated, and the reading at the distant staff (B in this example) is adjusted to what it *should* have been if the line of collimation was correct.
4. Adjusting the Line of Collimation: To correct the line of collimation, the horizontal crosshair is adjusted vertically using the diaphragm adjusting screws until the staff reading at the distant point matches the calculated correct reading. This process effectively brings the line of collimation into its correct horizontal plane relative to the level tube.

Why Other Options Are Incorrect

• Level tube: The adjustment of the level tube (or bubble tube) ensures that its axis is perpendicular to the vertical axis of the instrument. While important, the two peg test specifically checks the relationship between the line of sight and the level tube, primarily focusing on the line of collimation's horizontality, not the bubble tube's axis itself. Other tests are used for level tube adjustment.
• Cross hair ring: The cross hair ring holds the crosshairs. While adjusting the crosshairs is part of correcting the line of collimation, the two peg test isn't just about adjusting the ring itself. It's about ensuring the *line of sight* defined by the crosshairs and the objective lens is correct. The test provides the means to determine *how much* the crosshair needs to be adjusted to correct the line of collimation. The cross-hair ring is the physical component that is moved, but the error being corrected is in the line of collimation.
• Cross hair ring and line of collimation: While the cross hair ring is physically adjusted, the fundamental error being detected and corrected is the inclination of the line of collimation. Therefore, the most precise answer for what the test is done to correct is the line of collimation itself, as the adjustment of the cross hair ring is merely the *method* by which the line of collimation is corrected.

Therefore, the two peg test is done to correct the line of collimation of the level instrument, ensuring accurate elevation measurements in surveying.