1.15.4. Temporary adjustment of level

Firstly, the tripod stand is set up at a convenient height so that its head is horizontal (through eye judgement only). If the ground is flat, the three legs of tripod are kept at equal spacing to form equilateral triangle so that the levelling of the base is quick. The Level is then fixed on the tripod head by screws. The Level instrument is set up over a ground station by suspending the Plumb bob with the instrument base, and ensuring that the pointed tip of the Plumb bob touches the instrument station.

The first step in adjusting a level is to set up the tripod properly. You need to adjust its height so that the top is level with your eye. This means you should visually assess the level of the tripod head. If the ground is flat, you will place the three legs of the tripod evenly apart, forming an equilateral triangle, which aids in stability and quick leveling. Once the tripod is set, you attach the level instrument to the tripod using screws. Lastly, ensure the device is directly above a ground reference point by using a plumb bob, which helps in perfectly aligning the instrument with the ground point below it.

Imagine setting up a camera on a tripod. If the camera is set too high or low, your pictures will be misaligned. Similarly, by ensuring the level instrument is positioned correctly over its point using the plumb bob, you can be sure the survey measurements are accurate.

Secondly, the instrument is leveled approximately with the help of leveling screws till the circular bubble comes in the centre. Then, the bubble of the level tube is brought to the centre by using the designated screw. For approximate levelling of base, the telescope of the instrument is kept parallel to any two of the three levelling screws, and using these two screws the bubble should be brought in the centre of the levelling bubble tube. Once the bubble is centered, the telescope is brought perpendicular to its current position. Now, using the third screw, the bubble is brought exactly inside the circle. The telescope is now rotated 90º and the bubble is checked. If the bubble doesn’t stay in the center of the circle, the previous steps are repeated to bring it in the centre.

After setting up the tripod and instrument, the second step is to level the instrument. You will use leveling screws on the tripod to adjust the position until the bubble indicator in the level tube is centered. Begin by making sure that any two leveling screws are adjusted to bring the bubble to the center. Once the bubble is centered, rotate the telescope of the instrument to ensure it is perpendicular to its position. After this, utilize the third screw to get the bubble perfectly in the center. The process may require checking and re-adjusting by moving the telescope and checking the bubble again to ensure accuracy. Ideally, you would rotate the telescope 360° to confirm the bubble remains centered in all positions.

Think of a spirit level you use at home to hang a picture frame. You adjust it until the bubble is centered, indicating that your frame is straight. Similarly, careful leveling of the survey instrument ensures accurate readings when measuring distances and elevations.

Thirdly, focusing is required to be done in order to form the clear image through objective lens at the plane of the diaphragm and to view the clear image of through the eye-piece. The diaphragm is a cross-hairs at which the object is focused. These are available in different shapes. The focusing is carried out in two steps by removing parallax with proper focusing of objective lens and eye-piece. For focusing the eye-piece, screw attached to it is rotated till the diaphragm or cross-hairs are seen sharp and distinct. Focusing of cross-hairs might change from observer to observer, as it depends on the vision of the observer. For focusing the objective lens, the telescope is first pointed towards the Levelling staff, and the focusing screw is turned until the image of the Staff appears clear and sharp.

The last step is to focus the instrument, which ensures that images are clear for precise readings. The focusing process involves adjusting two parts: the objective lens and the eyepiece. The eyepiece has a screw to adjust how you see the crosshairs, allowing you to see them sharply. This setting can vary because different people perceive focus differently. For the objective lens, you will point the telescope at the leveling staff and turn the focusing screw until the image is sharp and doesn’t move relative to the crosshairs which means it provides the correct reference for taking measurements.

This process is similar to adjusting the focus on a camera or binoculars. When the lens is out of focus, everything appears blurry. However, fine-tuning it allows you to see clearly, ensuring your photographs or views are sharp and precise. In leveling, sharp focus means better accuracy in your measurements.

--