Linear measurement – Types, instruments and common errors
In engineering survey, the linear horizontal distance is to be measured to complete a survey. The measurement of this linear horizontal distance between two points on the earth surface is known as linear measurement. Linear measurement can be achieved by various instruments. The measurement type can be divided as,
electronic measurement method, depending on the type of instrument that is used to measure the distance.
Direct measurement method
Direct measurement is obtaining the measurement directly which means measuring the length directly using simple equipment and there is no need to solve hard equations..! In the Direct methods, the distance is actually measured during field work using a chain or a tape.
The following instruments are used while chaining:
- Ranging rods and offset rods
The chain is composed of 100 or 150 pieces of galvanized mild steel wire called links, joined together with oval rings and handles at both ends. The end of each link are bent into a loop and connected together by means of three oval rings. The ends of the chain are provided with brass handles for easy handling. The length of chain is measured from one handle to other handle.
Arrows or chain pins
They are also called as marking or chaining pins and are used to mark the end of chain during the process of chaining. They are made up of good quality hardened and tempered steel wire of 4mm in diameter. The arrows are made 400 mm in length. The one pointed part of an arrow is inserted into ground and the other is attached with ring.
Ranging rods or offset rods or ranging poles
Ranging rods or offset rods are round poles made up of wood or metal. These rods are used to range intermediate points of a survey and to set out straight lines on the field when the surveying length is long. The only difference between ranging rods and poles is the length. Ranging rods and offset rods are commonly occurs in 3m length but the ranging poles are available up to 8m length. The rods possess a metal point at its bottom and the rod or pole is painted with red & white or black & white successive combination. When the survey lines are too lengthy a flag with red/white/yellow colour will be attached to the top of the pole for easy identification.
The pegs are made up with wood and they used to mark the survey positions or terminals.
The size of the pegs (40 to 60 cm) depends on the type of survey work they are used for and the type of soil they have to be driven in. Although the pegs are driven into the ground 1/5 th of its part should be visible on the ground surface.
A string suspended with a weight at the bottom will be both vertical and perpendicular to any level plane through which it passes. The plumb consists of a specially designed weight and coarse string or special threads. At one end of the string the weight is affixed. Precisely machined and balanced bobs have pointed tips, and can be made of brass, steel, or other materials, including plastic.
Tapes are used for more accurate measurements. The tapes are classified based on the materials of which they are made of such as:
- Cloth or linen tape
- Fiber tape
- Metallic tape
- Steel tape
- Invar tape
Common instrumental errors in linear measurement
- Length other than standard
Tape manufacturers do not guarantee 100 ft steel tapes to be exactly 100.00 ft. An error due to the incorrect length occurs every time the tape is used.
- Temperature other than standard
Steel tapes are standardized at 68° F (20° C).
A temperature higher or lower will change the length of the tape. Temperature of the tape can be quite different from the air temperature measured.
If tension is greater than standard the tape will stretch. If less than standard tension is applied the tape will be shorter than standard.
A tape not supported along its entire length will sag. By applying correct tension the sag can be reduced.
- Poor alignment
This error occurs when one end of the tape is off-line or there is an obstruction in-line. The actual distance will be less than the measured dis
- Faulty marking
This error is random as the result of incorrect placement of chaining pins. Careful placement of chaining pins and then checking the measurement will reduce errors.
- Incorrect reading or interpolation
The error occurs when reading or noting the measurement clearly or in a hurry. Careful reading and using a small scale to determine the last figure will reduce errors.