The equipment used in the levelling process comprises optical levels and graduated staffs. Basically, the optical level consists of a telescope fitted with a spirit bubble or automatic compensator to ensure long horizontal sights onto the vertically held graduated staff.
Levelling staffs are made of wood, metal or glass fibre and graduated in metres and centimetres. The alternate metre lengths are usually shown in black and red on a white background. The majority of staffs are telescopic or socketed in three or four sections for easy carrying. Although the graduations can take
various forms, the type adopted in the UK is the British Standard (BS 4484) E-pattern type as shown in Figure 3.9. The smallest graduation on the staff is 0.01 m and readings are estimated to the nearest millimetre. As the staff must be held vertical during observation it should be fitted with a circular bubble.
The types of level found in general use are the tilting, the automatic level, and digital levels.
(1) Tilting level:
Figure. shows the telescope of the tilting level pivoted at the centre of the tribrach; an attachment plate with three footscrews. The footscrews are used to centre the circular bubble, thereby setting the telescope approximately in a horizontal plane. After the telescope has been focused on the staff, the line of sight is set more precisely to the horizontal using the highly sensitive tubular bubble and the tilting screw that raises or lowers one end of the telescope. The double concave internal focusing lens is moved along the telescope tube by its focusing screw until the image of the staff is brought into focus on the cross-hairs. The Ramsden eyepiece, with a magnification of about 35 diameters, is then used to view the image in the plane of the cross-hairs.
The cross-hairs which are etched onto a circle of fine glass plate called a reticule must be brought into sharp focus by the eyepiece focusing screw prior to commencing observations. This process is necessary to remove any cross-hair parallax caused by the image of the staff being brought to a focus in front of or behind the cross-hair. The presence of parallax can be checked by moving the head from side to side or up and down when looking through the telescope. If the image of the staff does not coincide with the cross-hair, movement of the observer’s head will cause the cross-hair to move relative to the staff image.
The adjusting procedure is therefore:
(1) Using the eyepiece focusing screw, bring the cross-hair into very sharp focus against a light background such as a sheet of blank paper held in front of the object lens.
(2) Now focus on the staff using the main focusing screw until a sharp image is obtained without losing the clear image of the cross-hair.
(3) Check by moving your head from side to side several times. Repeat the whole process if necessary.
Different types of cross-hair are shown in Figure . A line from the centre of the cross-hair and passing through the centre of the object lens is the line of sight or line of collimation of the telescope. The sensitivity of the tubular spirit bubble is determined by its radius of curvature (R) (Figure ); the larger the radius, the more sensitive the bubble. It is filled with sufficient synthetic alcohol to leave a small air bubble in the tube. The tube is graduated generally in intervals of 2 mm. If the bubble moves off centre by one such interval it represents an angular tilt of the line of sight of 20 seconds of arc. Thus if 2 mm subtends θ = 20, then: