In an engineering survey, the linear horizontal distance has to be measured. The measurement of this linear horizontal distance between two points on the earth's surface is known as linear measurement.
The study of surveying helps us determine the relative position of points, the height of buildings without actually measuring the settlement, etc. It is generally done prior to the construction of buildings.
Types
Linear measurements can be done by using different instruments. Based on the instrument used, it can be of different types.
Direct measurement method: It includes the use of instruments like a chain, tape, arrows, ranging rods, and offset rods
Optical measurement methods: It is done with a Tacheometer, which works on the principle of optics. The instrument is ideal when the ground is not suitable for surveying (uneven).
Electro-magnetic distance measurement (EDM) methods: It is one of the modern methods. Pulses are sent in the form of waves; time taken to hit the receiver and come back is measured; velocity is known; distance is calculated.
Let us know in detail learn about some of the instruments used in linear measurements.
Instruments Used
Linear measurements can be done by using different instruments. These include:
Chain
Tape
Arrows
Ranging rod and offset rods
Tacheometer
Electro-Magnetic distance measuring (EDM) device
Chain
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. Here the area is divided into a network of triangles, sides of various triangles are measured directly in the field with a chain or tape.
It is the simplest method of surveying and is suitable for small areas.
Tape
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.
Arrows
Arrows are used to facilitate chains and tape. They are used to mark the end of a chain. The one-pointed part of an arrow is inserted into the ground and the other is attached with
a ring.
Ranging rod
Ranging Rods are the vertical rods used to mark survey points having 2-3 m height. It is painted with bands of red & white, 20 cm each. These rods are used to range intermediate points of a survey.
Tacheometer
The tachometer is an instrument used to measure the vertical and horizontal distance between two points.
Electro-Magnetic Distance Measuring Device
EDM devices have the following features absolute circle reading, laser plummet, endless drives, 30 x magnifications, high-resolution LCD display. It is a very accurate method irrespective of terrain and works on electromagnetic waves. A pulse of radiation of a particular wavelength is sent.
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