Chain Surveying: The Fundamental for Linear Measurement
- The linear measurement is done by tape or chain in chain surveying.
- It is applicable for small, open areas having few simple details. Here details maybe classified as:
a) Hard details: Building, roads, walls
b) Soft details: Rivers, trees
c) Overhead details: Power and telephones lines
d) Underground details: Water and sewer pipes
i) Main station:
Main station is the point in chain survey at which the two sides of the triangle meet. These stations act like control point, or boundary of the survey area.
iI) Tie / Subsidiary station:
- Subsidiary station is the point on the line joining the two main stations. Interior details of the area are located with the help of tie stations.
iII) Main survey line:
- The line connecting the two main survey station is defined as the main survey line.
iV) Subsidiary/ Tie/ Auxiliary line:
- Auxiliary line is the line joining the two tie stations, and it is used for locating interior details of the survey area, which are located at a far distance from the main survey line.
v) Base line:
- Base line is the longest main survey line that passes through the centre of survey area.
- It is the most important survey line, as the location of the rest of the survey lines are fixed on the basis of this line only.
vI) Check/Proof line:
- Check line is provided to check the accuracy of fieldwork.
- The measured length of the check line and the computed line (from the plan) must be same.
- It is the distance of offset from the survey line.
- It may be perpendicular or oblique.
- It is the distance measured along the main survey line in the direction of the progress of work.
- It is generally used in linear construction projects.
well condition triangle:
- It is a triangle if all intersections of lines are clear for plotting purposes.
- If the angle between the lines is 30°-120°, a clear intersection is achieved.
- Triangle having angles less than 30° are considered as an ill conditioned triangle
equipment's used in chain surveying:
I) Surveying Chain:
- It is used for measuring distance where high accuracy is not required. The large links present in the surveying chain are made of galvanized mild steel wire of 4mm diameter.
- To enable the reading of fraction of chain length without difficulty brass tallies/ tags are provided at fixed intervals. [For example: in 10 m surveying chain, it is provided after every 1m interval]
types of chains:
1) Revenue chain: It is a 33ft. long chain, having 16 links, each link being 2(1/16) ft. long. It is used for the measurement of fields in a cadastral survey.
2) Gunter's chain: It is a 66 ft. long chain, having 100 links, each link being 0.6 ft or 7.92 inches long. Gunter chain is preferred if the linear measurements are required in furlong and miles.
3) Engineer's chain: It is a 100 ft. long chain having 100 links, each link being 1 ft. long. It is used if the distances measured are in feet and decimal.
4) Metric chain: The availability of these chains are in the lengths of 5, 10, 20 and 30 meters. Normally 20 m chain with 100 links and 30 m chain with 150 links are used.
- a) 1 mile = 80 Gunter's chain
- b) 1 acre = (10 Gunter's chain)?
- c) As per IS specification, every meter length of chain should be individually within ‡ 2mm, when measured under a tension of 80 N.
- d) The limit for the overall length of the chain should be as follows:
- i) 20 m ‡ 5mm for 20 m chain
- ii) 30 m ‡ 8 mm for 30 m chain
On testing, if a chain is found to be long, it can be adjusted by:i) Closing the joints of the rings if opened out
ii) Reshaping the elongated rings
iii) Adjusting the links at the end
iv) Replacing worn out rings
v) Removing one or more small circular rings
- The tapes are easy to handle and more precise than chains.
The tapes are classified as follows based on the material used:
a) Cloth / Linen tape:
- It is made up of synthetic material.
- Its length varies from 10-30 m, and its width varies from 12-15 mm.
- It gets shrunk in the presence of moisture.
- It gets damaged when stretched more, and it is likely to twist during measurement.
b) metallic tape:
- It is linen tape reinforced with brass or copper wire.
- Its accuracy is more than linen tape.
c) steel tape:
- It is more accurate than metallic tape.
- Its length varies from 1-50 meters and width from 6 - 10 mm.
- It is affected by temperature and moisture.
d) invar tape:
- It is made up of an alloy of nickel and steel (36% nickel and 64% steel).
- It is highly precise.
- It is less affected by temperature (α = 0.122×10-6/ °C).
- It is soft and deforms easily.
- There are used to mark definite points on the ground from which measurement are to be taken, or any instruments are to be set.
- While measuring the length of the long line, chain/ tape is to be laid number of times and the position of the end is marked with the help of arrows.
- Its length varies from 25 cm to 50 cm, and the most common length is 40 cm. The one end of the arrow is made sharp, and the other end of the arrow is bent into a loop or circle for the facility of carrying it.
V) ranging rod:
- Intermediate points lying on the straight line used to join the end stations can be located using a ranging rod.
- The material of this rod is generally well-seasoned, straight grained timber and cross-sectional shape consist of circular or octagonal patterns in 3cm nominal diameter.
- The length of the rod is kept between 2m to 3m.
- For clear identification of measurement, alternate bands of red/ black and white colour having 200 mm length are imprinted on them.
- Major difficulty in measurement occurs when the observation point lies at more than 200m distance. To overcome this, each rod should have a flag (30 to 50 cm square) tied near its top, having colour any of red, white or yellow.
Note: Ranging- It is the method in which a which a number of point are located on the long survey line
VI) offset rod:
- The offset rod differs from the ranging rod as they consist of two narrow vertical slots at a right angles to each other at eye level and a hook or opening at the top of the rod. The offset line is aligned using the offset rod.
Q. What is the purpose of providing a notch or hook at the top of the offset rod ?Sol: The hook facilitates pulling and pushing the chain through hedges and other obstructions.
VII) cross Staff:
a) It is the simplest instrument used for setting out offsets to the chain Line from a given point known as "OPEN CROSS STAFF" .It consists of two slits providing two lines of sight mutually at right angles
- It is also used for setting out right angles at a given point on the survey line.
b) In order to set out angles in multiple of 45° "FRENCH CROSS STAFF" can be used, the accuracy of which is less than cross staff.
c) In order to set out any desired angle from the survey line "ADJUSTABLE CROSS STAFF"can be used.
It consists of two cylinders of equal diameter which can rotate over each other. The lower cylinder consists of the main and sub-main scale, and the upper cylinder consists of the vernier scale and a slit to provide a line of sight.
VIII) optical square:
- It is a small instrument used to set out right angles and is more convenient and accurate than cross staff.
- The two mirrors, Horizontal (H) and Index (l) are placed at an angle of 45°.
- The fully silvered mirror 'l, is placed opposite to the opening F & the mirror H, which is half silvered. (silvered on the top and unsilver at the bottom) It is placed, facing the opening E.
- It works on the principle of "double reflection" i.e., If a ray of light is reflected or undergoes reflection successively than the angle between the incident and the reflected light is twice the angle between the reflecting surface.
Note: Instead of using two mirrors, a prism can also be used having two reflecting sides at an angle of 45° and the unit is termed as "PRISM SQUARE".
Q. What is the difference between a prism square and an optical square?Sol: The prism square works on the same principle as that of optical square, the only difference is that in prism square, no adjustment is required since the angle between the reflecting surfaces (i.e.45°) cannot vary.
iX) Simple clinometer:
- The instrument is in the shape of a semi-circle, a light plumb bob suspended from its centre. It is used for the measurement of the angle of slope of ground.
- The flat base is attached to the semi-circle to lay the clinometer on the surface whose slope is to be measured.
x) Line ranger:
- It is a device which is used to set intermediate points on the survey lines.
- It consists of two right angled isosceles triangular prism placed one over each other.
- The diagonals of these two prisms are painted silver to form the reflecting surface.
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