Linear measurements is defined as a measurement of length. It includes measurements of length, diameters, heights and thickness.
Methods of Linear Measurement
There are various methods of making linear measurements and their relative merit depends upon the degree of precision required. These can be mainly divided into three types:
- Direct Measurements
- Measurements by Optical Means
- Electronic Methods
Methods in reconnaissance for measurement of horizontal distance
Commonly used methods in reconnaissance for measurement of horizontal distance are pacing, Passometer, Pedometer, Odometer, Measuring Wheel, Speedometer, Perambulator, Chaining etc.
- Pacing or Stepping: For rough and speed work, distances are measured by pacing, i.e. by counting the number of walking steps of a man. The walking step of a man is considered as 2.5 ft or 80 cm. This method is generally in the reconnaissance survey of any project. It may also be used to roughly check the distances measured by other means.
- Passometer: Passometer is an instrument shaped like a stop watch and is carried in pocket or attached to one leg. The mechanism of the instrument is operated by motion of the body and it automatically registers the number of paces, thus avoiding the boredom and strain of counting the paces, by the surveyor. The number of paces registered by the passometer can then be multiplied by the average length of the pace to get the distance.
- 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 paces.
- Odometer: Odometer is a wheel fitted with a fork and handle. The wheel is graduated and shows a distance per revolution. There is a dial which records the number of revolutions. Thus the distance can be ascertained.
- Speedometer: The speedometer is used in automobiles for recording distances, if the route is smooth and its working is similar to as odometer.
- Chaining: Chaining is a term 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. The various instruments used for the determination of the length of line by chaining are: Chain or tape, Arrows, Pegs, Ranging rods, Offset rods, Plumb bob etc.
Chains are formed of straight links of galvanized 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 with brass handle at each end with revolve joint, so that the chain can be turned without twisting. Tallies are provided at every 10 or 25 links for facility of counting. The length of a link is the distance between the centers of two consecutive middle rings, while the length of the chain is measured from the outside of one handle to the outside of the other handle.
Various Types of Chain
- Metric Chains: Metric chains are generally available in lengths of 5, 10, 20 and 30 metres. To enable the reading of fractions of a chain without much difficulty, tallies are fixed at every metre length for chains of 5 m and 10 m lengths and at every five-metre length for chains of 20 m and 30 m lengths. In the case of 20 m and 30 m chains, small brass rings are provided at every metre length, except where tallies are attached.
- Gunter’s Chain or Surveyor Chain: Gunter’s Chain or 66 ft. Chain: Divided into 100 links, each link is of 0.66 ft. or 7.92 inches. Also called Surveyor’s chain. Engineer’s chain and Gunter’s chain are commonly used in our country. It was previously used for measuring distance in miles and furlongs (10 Gunter’s chain = 1 furlong 80 Gunter’s chain = 1 mile).
- Engineer’s Chain: The engineer’s chain is 100 ft. long and consists of 100 links, each link being 1 ft. long. Tallies are provided at every 10 links, then central tally being round.
- Revenue Chain: The revenue chain is 33 ft long and consists of 16 links each link being 2.0625 ft long. It is mainly used in cadastral survey.
- 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 band are available in lengths of 20 or 30 metres. It is graduated in meters, decimeters and centimeters on one side and has 0.2 m links on the other. It is used in projects where more accuracy is required.
Methods of chaining on sloping ground
- Direct Method
- Indirect Method (1) by measuring the slope with clinometer (2) by providing hypotenusal allowance (3) by knowing the difference of level
Tapes are available in a variety of materials, lengths and weights. The different types of tape are:
- Cloth or Linen Tape: These are closely woven linen or synthetic material and are varnished to resist the moisture. These are available in 10 to 30 m in length and 12 to 15 mm in width.
- Metallic Tape: It is a linen tape with brass or copper wires woven into it longitudinally to reduce stretching. As it is varnished, the wires are not visible. These are available in 20-30 m length. It is an accurate measurement device and is commonly used for measuring offsets. As it is reinforced with wires, all the defects of linen tapes are overcome.
- Steel Tape: These are 1 to 50 m in length and are 6- 10 mm wide. At the end of the tape a brass ring is attached, the outer end of which is zero point of the tape.
- Invar Tape: This is made of an alloy of nickel (36%) and steel, having very low coefficient of thermal expansion (0.122 x 10-6 / 0C). These are available in lengths of 30, 50 and 100 m and in a width of 6 mm.
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 to 60 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.
Arrows (Chain pin): Arrows are made of stout steel wire. 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 enameled. The length of arrow may vary from 25 cm to 50 cm (generally 40 cm). One end of the arrow is made sharp and other end is bent into a loop or circle for facility of carrying.
Ranging rods have a length of either 2 m or 3 m, the 2 metre length being more common. They are combined 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 cm depth 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 line. They are circular or octagonal in cross-section of 3 cm 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 cm square, tied on near its top.
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 shoe at one end, and provided with a notch or a hook at the other. The hook facilitates pulling and pushing the chain through hedges and other obstructions. The rod is mainly used for measuring rough offsets nearby. It has also two narrow slots passing through the center of the section, and set at right angles to one another, at the eye level, for aligning the offset line.
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 table and a variety of other surveying instruments.