LINEAR MEASUREMENT IN SURVEY

 Differents Methods of Linear Measurements :-

There are various methods of making linear measurements and their relative merit depends upon the degree of precision required.

They can .be mainly divided into three heads:-

• Direct Measurements
• Measurements by optical means
• Electro-magnetic methods

In the case of direct measurements, distances are actually measured on the ground with help of a chain or a tape or any other instrument. In the optical methods, observations are taken through a telescope and calculations are done for the distances, such as in tacheometry or triangulation. In the electro-magnetic methods, distances are measured with instruments that rely on propagation, reflection and subsequent reception of either radio waves, light waves or infrared wave.

DIRECT MEASUREMENTS

The various methods of measuring the distances directly are as follows :-

1. Pacing Measurement with Passometer Measurement with Pedometer
2. Measurement by odometer and Speedometer
3. Chaining

1) Pacing  :-  Measurements of distances by pacing is chiefly confined to the preliminary surveys and explorations where a surveyor is called upon to make a rough survey as quickly as possible. It may also be used to roughly check the distances measured by other means. The method consists in counting the number of paces between the two points of a line.The length of the line can then be computed by knowing the ·average length of the pace.

2) Passometer :-  Passometer is an instrument shaped like a watch and is carried in pocket or attached to one leg. The mechanism of the instroment is operated by motion of the body and it automatically registers the number of paces, thus avoiding the monotony and strain of counting the paces, by the surveyor.

3) Pedometer :-  Pedometer is a device similar to the passometer except that, adjusted to the length of the pace of the person carrying it, it registers the total distance covered by any number of pace.

4)  Odometer and Speedometer :-  he odometer is an instrument for registering the number of revolutions of a wheel. The well-known speedometer works on this principle.  The odometer is fitted to a wheel which is rolled along the lirie whose length is required. The number of revolutions registered by the odometer can then be multiplied by the circumference of the wheel to get the distance.

5) Chaining  :-  Chaining is a renn which is used to denote measuring distance either with the help of a chain or a tape and is the most accurate method of making direct measurements. For work of ordinary ptecision, a chain can be used, but for higher precision a tape or special bar can be used. The distances determined by chaining form the basis of all surveying. 

INSTRUMENTS FOR CHANING

The various instruments used for the detennination of the length of line by chaining
are as follows:-

1. Chain or Tape
2. Arrows
3. Pegs
4. Ranging rod
5. Offset rod
6. Plumb bob

1. CHAIN OR TAPE

Chains are fanned of straight links of galvanised mild steel wire bent into rings at the ends and joined each other by three small circular or oval wire rings. These rings offer flexibility to the chain. The ends of the chain are provided whh brass handle at each end with swivel joint, so that the chain can be turned without twisting.

Following are various types of chains in common use:- 

• Metric Chain
• Gunter's Chain or Surveyor's Chain
• Engineer's Chain
• Revenue Chain
• Steel band or band chain

 1) Metric Chain :- fter the introduction of metric units in India. the metric chains are widely used.         

     Metric chains are generally available in lengths of 5. 10, 20 and 30 merter.

2) Gunter's Chain or Surveyor's Chain  :- A Gunter's chain or surveyor's chain is 66 ft. long and consisiS of 100 links. each link being 0.6 ft. or 7.92 inches long. The length of 66 ft. was originally adopted for convenience in land measurement since 10 square chains are equal to I acre. Also. when linear measuremeniS are required in furlongs and miles, it is more convenient since 10 Gunter's chains = 1 furlong and 80 Gunter's chains = 1 mile.

3) Engineer's Chain :-  The engineer's chain is 100 ft. long and consisiS of 100 links, each link being 1 ft. long. At every I0 links, brass tags are fastened, with notches on the tags indicating the number of 10 link segmeniS between the tag and end of the chain. The distances measured are recorded in feet and decimals.

4)  Revenue Chain :-  The. revenue chain is. 33 ft. long and consist of 16 links,  The chain is mainly used for measuring fields in cadastral survey.

5) Steel band or band chain  :-  The steel band consists of a long narrow strip of blue steel, of uniform width of 12 to 16 mm and thickness of 0.3 to 0.6 mm. Metric steel bands are available in lengths of 20 or 30 m. It is divided by brass studs at every 20 em and numbered at every metre. The first and last links (20 em length) are subdivided into em and mm. Alternatively• in the place of putting brass studs, a steel band may have graduations etched as metres. decimetres and centimetres on one side and 0.2 m links on the other.

2) ARROWS

Arrows or marking pins are made of stout sreel wire. and genera1ly. 10 arrows are supplied with a chain. An arrow is inserted into the ground after every chain length measured on the ground. Arrows are made of good quality hardened and tempered steel wire 4 mm in diameter, and are black enamelled. The length of arrow may vary from· 25 em to 50 em, the most common length being 40 em. One end of the arrow is made sharp and other end is bent into a loop or circle for facility of carrying.

3) PEGS

Wooden pegs are used to mark the positions of the stations or terminal points of a survey line. They are made of stout timber, generally 2.5 cm or 3 cm square and 15 cm long, tapered at the end. They are driven in the ground with the help of a wooden hammer and kept about 4 cm projecting above the surface.

4) RANGING ROD

Ranging rods have a length of either 2 m or 3 m, the 2 meter le;ugili being more eommon. They are shod at the bottOm with a heavy iron point, and are painted in alternative bands of either black and white or red and white or black, red and white in succession, each band being 20 em deep so that on occasion the rod can be used for rough measurement of short lengths. Ranging rods are used to range some intermediate points in the survey tine. They are circular or octagonal in cross-section of 3 em nominal diameter, made of well-seasoned, straight grained timber. The rods are almost invisible at a distance of about 200 metres; hence when used on long lines each rod should have a red, white or yellow flag, about 30 to 50 em square, tied on near its top.

5)  OFFSET ROD

An offset rod is similar to a ranging rod and has a length of 3 m. They are round wooden rods, shod with pointed iron shae at otle end, and provided with a notch or a hook at the other. The hook facilitates pulling and pilshing the chain through hedges and other obstruction.

Butt rod :-  A butt rod is also used for measuring offsets, but it is often used by building surveyors or architects. It generally consists of two laths, each of 1 yard or 1m in length loosely riveted together. The joint is also provided with a spring catch to keep the rod extended.

6) PLUMB BOB

While chaining along sloping ground, a plumb-bob is required to transfer the points to the ground. It is also used to make ranging poles vertical and to transfer·points from a line ranger to the ground. In addition, it is used as centering aid in theodolites, compass, plane rable and a variety of other surveying instruments.

                                                           

                                                                 FIG :- Plumb Bob