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Chapter -II
Measurement of Distance
Accessories for measurement of distance:-
1.Ranging rod
2. Tape
3. Chain
4. Arrow
5. реg
6. Plumb bob
1. Ranging rod.
Rods which are used for ranging a line is called ranging rod such rods are made of seasoned timber, bamboo or GI pipe of 3cm diameter. They are generally circular in section. The rods are 2m-3m in
Length with heavy pointed iron in the lower end. The rod is either colored black and white, so that it is visible from long distance. The length of each colored
section it is 20 cm.
2.Tape
It is the most common accessories used in measurement of distance. There are four type of tapes:
a)cloth (Linen tape)
b)Metallic tape
c)steel tape
d)Invar tape
1)cloth / Linen tape
such tape are made of closely woven wire and it is varnished to resist moisture. They regenerally 10 - 15m in length and 12 mm -15mm In width. Linen tapes are used to measure offsets. They are easily affected by moisture.
b. Metallic tape.
When linen tape is reinformed with brass or copper wire to prevent streetching it is called metallic tape. It is more durable than linen tape. Metallic tape are available in 15m, 20m, 30m / m length and it has 15 mm width.
C. Steel tape
steel tape is made of steel va ribbon of width varying from 6mm -16mm. They are. available in lom, 20m, 30m and 50m in length.
d) Invar tape
Invar tape is made up alloy of steel 64% and nickel (36%). It's thermal coffecient is very low. Therefore it is not affected by change in temperature. It is used in places where maximum precion is requried.
3)Chain
Chain is prepared with glavanized mild steel wire of umm diameter. The end of chain are bent to form loops. then the pieces are connected together with the help of oval rings which makes the chain flexible. The two end of chain are provided with brass handle.
Type of chain
A. Metric chain
→ Metric chain are available in the length of 20m and 30m which are divided into loo links, each link is of 0.2m. The 30m chain is divided into 100 links. each link is at 0.3 m.
B-Engineering chain
→The engineering chain is 100ft long and divided into 100 links, each links is 1 ft.
C.Gunter's chain
→ The gunter's chain is 66 ft long and divided into 100 links - Each link is of 0.66 ft.
D. Revenue chain.
Revenue chain is 33 ft long and divided
Into 16 links. So, that each link is 2.0625ft
4.Arrow
Arrow are made up of tempered steel wire 4mm diametre. one end of arrow is bent into the ring shape of 50 mm diametre and another end pointed. The overal length of arrow is 400mm Arrow are used for marking stations, reberence point counting number at chain etc.
5.Peg
While measuring the chain line wooden pegs are used to mark the end of chain line. Normally pegs denotes the station point. The peg are made of hard wood of 25*25 mm Cross section. 150mm length with one end point as shown in figure
6 Plumb bob
-It is mostly used instrument in surveying.
-It is used for verticality measurement.
Correction of tape or chain
-Temperature correction
The length of tape may increase or decrease due to rise and fall in temperature during measurment in the field. So, correction for temperature is requride, whicb is given by the expression bellow :-
Ct=α(Tm - Tnot) L
Where
Ct= temperature correction in m
α= coefficient of thermal expansion
Tm= temperature during measurement
T not= Temperature at which tape was standardised
L =length of tape
-Pull correction
The pull apird during the measurement maybe either more or less than at which tape was standardised. The expression for pull correction is :-
Cp = (Pm - P not * l ) / A * E
Where
Cp= pull correction
Pm =pull applied during measuring
P not= pull at which the tape was standardised
L= length of tape
A= cross-section area of tape
E =modulus of elasticity
- Sag correction
Sag correction is necessary when the measurement is taken with the tape in supensim(in the form of catenary)
It is given bellow:-
Cs=lw^2/24n^2Pm^2
Where
W =weight of tape in kg
L =length of tape
Pm= pull applied during measurement
n= number of span
[*Sag correction is always negative]
The actual length of tape is L'=L+Ct+Cp-Cs
Ranging:-
The process of established intermediate point on a straight line between two end point is known as ranging. Ranging must be done before a survey line is chanied.
Ranging is of two type:-
1.Direct ranging
2.Indirect ranging
1.Direct ranging
If the first and last point of survey line is intervisible than this method is possible when intermediate ranging rods are fixed on a straight line by direct observation from end station the process is called direct ranging.
Process of direct ranging
1. Assume that A and B are two end station of chain line where ranging rod are already fixed.
2. Suppose that another ranging rod is to be fixed at intermediate point C on chain line so that point A B C are in straight line.
3. The surveyor position himself approximately at 2m behind station A and looks along line AB. The surveyor direct the assistant to move ranging rod right or left to the line until the 3 rods come in exactly on same line.
4. When surveyor is satisfied he signals assistant to fix the rod on point C.
-Indirect ranging
The ranging in which first point and last point of a survey line are not intervisible due to undalating ground, hills etc is called indirect ranging. Minimum 4 ranging rods are required for indirect ranging.
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