equipment

the equipment used in the leveling process comprise 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 

leveling 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 graduation can take various froms the type adopted in the uk is the british standard (BS 4484 ) E - pattern type , the small graduation on the staff is 0,01 m and reading are estimated to the nearest millimetre , as the staff must be held vertical during observation it should be fitted with a circular bubble , 

optical levels

types of level found in general use are the tilting , the automatic level , and digital levels.

(1) tilting level

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 the double concave internal focusing lens is moved alone the telescope tube by its focusing screw until the images of the staff is brough 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.

leveling procedure

titling level

vertical control 

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 observation . this process is necessary to remove any cross-hair parallx 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 checked by moving the head from 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 images . 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 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 next . 

Aline from the centre of the cross-hair and passing through the cente of the object lens is the of sight or line of collimation of the telescope ,

The sensitivity of the tubular spirit bubble is determined by its radius of curval (R) 

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 intrval it represent an angular tilt of the line of sight of 20 seconds of arc . thus if 2 mm subtends 0 = 20'' , then ;

R = ( 2 mm × 206 265 ) / 20''  =  20.63 m 

The bubble attached to the titling level may be viewed directly or by means of coincidence reading system , in this latter system the two ends of the bubble are viewed and appear as shows the image when the bubble is off centre this method of viewing the bubble is four or five times more accurate than direct viewing .

The main characteristics defining the quality of telescope are its powers of magnification . the size of its field of view , . the brightness of the image formed and the resolution quality when reading the staff .

cross - hairs

tubular bubble

bubble coincidence reading systems : (a) bubble central (b) bubble off centre

all these are a function of the lens systems used and vary accordingly from low-order builder's levels to very precise geodetic levels .

Magnification is the ratio of the size of the object viewed through the telescope to its apparent size when viewed by the naked eye . surveying telescopes are limited in their magnification , the greater the effect of heat shimmer , on - site vibration and air turbulence . telescope magnification , the greater the effect of heat fhimmer , on - site vibration and air turbulence. telescope magnification lies between 15 and 50 times . 

the field of view is a function of the angle of the emerging rays from the eye through the telescope , and varies from 1 ُ to 2 ُ . image brightness is the ratio of the brightness of the image when viewed through the reticule . of an internal focusing telescope loses about 40% of the light . the resolution quality or resolving power of the telescope is its ability to define detail and is independent of magnification , 

it is a function of the effective aperture of thr object lens and the wavelenght - of light and represented in angular units , it can be computed from P radians = 1.2 / effective aperture )

(2) using titing level

(1) set up instrument on a firm . secure tripod base.

(2) centraize the circular bubble using the footscrews or ball and socket arrangement .

(3) eliminate parallax.

(4) centre the vertical cross-hair on theleveling staff and clamp the telescope. use the horizontal slow - motion screw if necessary to ensure exact alignment.

(5) focus onto the staff.

(6) carefully centre the tubular bubble ising the titling screw .

(7) with the staff in field , note the staff reading (1.045) and record it operation (4) to (7) must repeat for each new staff reading .