LEVELLING - Civil Engineering psc short notes

Branch of science dealing with 

i. Finding elevation of a point wrt other point 

ii. Establishing a point at a given elevation 

TERMS 

i. LEVEL SURFACE : surface parallel to mean spheroidal surface of earth , perpendicular to plumb line 

ii. LEVEL LINE : a line liying on level surface 

iii. HORIZONTAL SURFACE : a surface tangential to the level surface 

iv. HORIZONTAL LINE : a line on Horizontal surface 

v. DATUM : any arbitrarily assumed level surface or line from which vertical distances are measured 

vi. ELEVATION /RL/ SPOT LEVEL : The elevation of a point is its vertical distance above or below the datum. 

vii. MEAN SEA LEVEL : average sea level for 19 years. In india Mumbai is taken as datum 

viii. BENCH MARK : a fixed reference point at known elevation. 

a) GTS benchmark :established by survey of India , high precision 

b) Permanent benchmark : established by state gov. wrto gts. 

c) Temporary benchmark : established at the end of the day work. 

d) Arbitrary benchmark : for smaller work any permanent point is assumed as benchmark. 

METHODS OF LEVELLING 

1. SPIRIT LEVELLING : a horizontal surface is established using a spirit level wrt vertical distance are measured . Eg dumpy level 

2. BAROMETRIC LEVELLING : deference in elevation found using difference in pressure 

3. HYPSOMETRIC LEVELLING : deference in elevation found using boiling point of water 

4. INDIRECT LEVELLING / TRIGONOMETRIC LEVELLING : using vertex angle and horizontal distance 

TYPES OF LEVELLING 

1. DIFFERENTIAL LEVELLING : find level difference between two point irrespective of their location 

i. Fly levelling : to establish a benchmark from another benchmark 

ii. Check levelling : to check the accuracy of the levelling work 

2. PROFILE LEVELLING / LONGITUDINAL LEVELLING : find the elevation of points along a plotted center line 

3. CROSS SECTIONING : levels taken to either sides of the plotted centerline in transverse direction 

4. RECIPROCAL LEVELLING : 

• if instrument can't be set up midway between the points ( river ) 

• Most accurate method 

• Error due to curvature, refraction, instrumental errors, line of sight, can be eliminated 

• Errors are eliminated by taking mean of reading taken from both stations Profile & c/s leveling done simultaneously for road construction 

LEVELLING INSTRUMENTS 

1. LEVEL 

a) Dumpy level 

• The telescope is rigidly fixed to its supports 

• Cannot be rotated about its longitudinal axis, nor can it be removed from its supports 

• The Dumpy level has greater stability of adjustments than the reversible type levels 

b) Y level 

• The telescope is mounted on 2 y shaped columns and clamps 

• Can be removed and reversed 

c) Reversible level 

• Has the rigidity of dumpy level and reversibility of y level 

d) Tilting level 

• The line of sight can be tilted using a tilting screw 

PARTS OF A LEVEL 

1. TELESCOPE 

• Has 2 compound lenses - objective and eye piece 

• Has a cross hair in between, close to the eye piece 

• Focusing 

 process of bringing clear image of the object in the plane of crosshair 

 Focusing objective : brings the clear image of object to the crosshair 

 Focusing of eye piece : bring the image to point of distinct vision of eyes 

 Both should be simultaneously focused to remove parallax 

 parallax causes apparent movement of image wr. to crosshair 

DEFECTS IN TELESCOPE 

i. Spherical abrasion 

• Rays passing through edges converge nearer to the lens /// 

• Absence of spherical abrasion : aplanatisation 

ii. Chromatic abrasion 

• Red rays converge far to the lens an violet converge near 

• Absence of chromatic abrasion : acromatisation 

• Achromatic lens is used to reduce CA : convex lense 

OPTICAL CHARACTERISTICS 

1. Definition : ability to produce sharp image 

2. Resolving power : ability to produce distinguishable images of object separated by small angular distance 

3. Magnification : ratio of angle subtended at the eye to that subtended at the object 

4. Field of view / size of field : circular area seen through telescope , inversely prop to magnification 

5. Brightness / illumination 

• Inversely prop to no. of lens and magnification 

2. LEVELLING HEAD 

• Consist of 2 plates, upper tribach and lower tribach 

• 3 or 4 foot screws are provided between the plates 

3. TRIPOD 

Internal focusing telescope 

• Focusing is done by a double concave lens inside 

• Diagonal eye piece is used if line of sight is inclined to the axis of telescope 

External focusing telescope

• Focusing is cone by the relative motion of inner and outer tube 

 2. LEVELLING STAFF 

• Least count 5 mm 

• Types 

i. SELF-READING STAFF : 

• readings can be taken by the observer 

a. Solid staff : 3m long , single piece of wood 

b. Folding staff : 4m long, can be folded at middle 

c. Telescopic staff : 5m long, top segment solid and other segments hollow 

ii. TARGET STAFF 

• only target can be seen by the observer 

• Reading is taken by the person holding the target 

• Target is a circular disk divided into 4 quadrants and slot provided at the center with a reading 

ADJUSTMENTS OF A LEVEL 

1. PERMANENT ADJUSTMENT 

i. Axis of bubble tube should be parallel to line of sight 

ii. Axis of bubble tube should be perpendicular to vertical axis 

iii. Horizontal crosshair should be perpendicular to vertical axis 

2. TEMPORARY ADJUSTMENT 

i. Setting up of the level 

ii. Levelling up 

iii. Elimination of the parallax 

TERMS 

• STATION : point where the levelling staff is placed, point whose elevation is to be determined 

• THE LINE OF COLLIMATION / LINE OF SIGHT: the line joining the intersection of the crosshairs to the optical center of the object glass and its continuation. 

• AN AXIS OF TELESCOPE : a line joining the optical center of the object glass to the center of the eye- piece 

• AN AXIS OF LEVEL TUBE : imaginary line tangential to the curve of the tube at its middle point. It is horizontal when the bubble is centered. 

• THE HEIGHT OF INSTRUMENT (H.I) : is the elevation of the plane of collimation or the line of sight when the instrument is correctly levelled 

• A BACK SIGHT (B.S ) : 

• staff reading taken on a point of known elevation (bench mark). 

• It is also called a plus sight. 

• It is the first staff reading taken after the level is set up and levelled. 

• A FORESIGHT (F.S) : 

• Last sight taken from a station before shifting the instrument. 

• It is also termed as a minus sight. 

• AN INTERMEDIATE SIGHT (I.S) : any other staff reading other than BS and FS 

• CHANGE POINT / TURNING POINT : point where both foresight and back sight are take

METHOD OF LEVELING 

Height of instrument ( HI ) / height of collimation

• Commonly used in differential levelling 

• Less calculation 

• No check on intermediate sight, mistakes remains undetected 

• ΣBS - ΣFS = last RL - first RL 

Rise and fall 

• Commonly used in profile levelling 

• More calculations 

• Provides check for IS, mistakes can be easily detected 

• ΣBS - ΣFS = last RL - first RL = Σ rise - Σ fall 

• Used in gov. works 

Permissible error in levelling 

1. Rough levelling : ± 96 √k 

2. ordinary levelling : ± 24 √k 

3. Accurate levelling : ± 12 √k 

4. Precise levelling : ± 6 √k or ± 4 √k 

5. International geodetic survey : ± 1 √k

CORRECTION FOR LEVELLING 

1. CURVATURE CORRECTION 

• Due to curvature of earth staff reading becomes higher 

• Due to curvature effect object appear lower or smaller 

• Error +ve , correction - ve 

• Correction : -ve 0.0785 d² 

2. DUE TO REFRACTION 

• Due to refraction staff reading decreases 

• Due to refraction effect object appear higher than they really are 

• Error +ve, correction -ve 

• Refraction correction = 1/7 curvature correction 

• Correction : +ve 0.0112 d² 

3. COMBINED CORRECTION 

• Due to combined effect object apper lower or smaller 

• Correction = Curvature correction + Refraction correction = -ve 0.0673 d²