What are the different types of Level in Civil Engineering Survey

LEVELLING

* Levelling is the art of determining the elevations of the given point above or below a datum line or

establishing given points of required heights above or below the datum line.

* The point or the surface with respect to which levels of other points or planes are calculated is

called datum or datum surface.

* Mean sea level (MSL) is the average sea level for all stages of the tides which is established after

observing tides for a long period of 19 years. In India MSL is the level established at Karachi,

presently in Pakistan.

* The levels of various points taken as height above the datum surface are known as reduced levels

(RL).

* Benchmark is a relatively permanent point of reference whose elevation with respect to some

assumed datum is known. There are four types of bench marks; namely, GTS (Great Trignometric

Survey) benchmarks, permanent benchmarks, arbitrary benchmarks and temporary benchmarks.

* Methods of levelling employed are:

1. Direct levelling (spirit levelling)

2. Barometric levelling

3. Hypsometric levelling

4. Trigonometric (indirect) levelling

5. Stadia levelling

* Levelling instrument consists of:

1. A telescope to provide line of sight

2. A level tube

3. Levelling head for levelling the tube

4. A tripod to support the instrument

• Type of levels available are

1. Dumpy level

2. Y-level

3. Cooke’s reversible level

4. Cushing’s level

5. Telting level

6. Auto level

• Levelling staff:

A levelling staff is a straight rod with rectangular section having graduations, the foot of the staff

represents zero reading.

• Types of levelling staf s: Self-reading staff and target staff. There are three types of self-reading

staff are: solid staff–single piece of 3 m.

Folding staff–two piece, 2 + 2 m telescopic staff–three piece 2 + 1.25 + 1.25 or 2+ 1.5 + 1.5 m

Target staff may be single piece or extendable type.

• Parts of telescope: Telescope consists of objective, eyepiece, diaphragm and body with focussing

device. It may be of external focussing type or internal focussing type.

* Temporary adjustment of level consists of setting, levelling and focussing.

* The following terms are used in levelling:

1. Station: The point over which the level is set.

2. Height of instrument: It is the elevation of plane of sight. It is known as plane of collimation

also.

3. Back sight: It is the reading taken on a point of known elevation (bench mark or change point)

to find plane of collimation.

4. Intermediate sight (IS): Sight taken on points to establish their reduced level. They are the

sights taken after back sight and before foresight.

5. Foresight: This is the last reading taken from an instrument station before shifting.

* Types of levelling: Various types of levelling are simple, differential, fly, profile, cross sectional

and reciprocal levellings.

* Methods of reducing levels are height of instrument method and rise and fall method.

* In case of height of instrument methods, there is no check for the levels obtained by intermediate

sight whereas rise and fall method gives check for them also.

* Curvature corrections: If the sight distance is more, the curvature correction is required because

vertical line at the staff point is not at right angle to horizontal line from instrument point. This

correction is equal to where d is the measured distance and R is radius of the earth.

* The line of sight is refracted since when distance is large, it is travelling through denser to thinner

air. For average condition, it is assumed to have a radius of curvature 7 times that of the earth.

Hence, correction for refraction is + .

* Total correction due to curvature and refraction is

= = – 6.728 × 10

–8 d

2 metres

* If P is the position of observation which is h metres above mean see level, distance of visible

horizontal in terms of metres after taking into account curvature and refraction, i.e., given by d =

3855.3 .

* By balancing back sights and foresights, the following errors are eliminated:

1. Error due to non-parallelism of the line of collimation and axis of bubble tube.

2. Error due to curvature and refraction.

* Errors in levelling may be grouped into the following three categories.

1. Instrument errors

2. Natural errors

3. Personal errors

* After carrying out levelling, if fly levels are taken and ended on the same benchmark closing error

can be found. The permissible closing error is

e =

where c = constant and k = Distance in km

Permissible value of k for different works are:

1. Preliminary survey ± 100

2. Construction survey ± 24

3. For establishing benchmark ± 12

4. Geodetic survey ± 1

Trigonometric Levelling

* This is the method of levelling in which difference between elevation of two points is found by

measuring vertical angles and horizontal distances.

* If base of the object is accessible, the distance of the object and vertical angle to the object may be

measured to find the RLof the object.

* If base line is inaccessible single plane method or double plane method may be used to find RL of

inaccessible object.