Linear measurement: Instruments for measuring distance and ranging
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,
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.
Types of Surveying Chains
Following are the various types of chain in common use:
- Metric chains
- Steel band or Band chain
- Gunter's chain or surveyors chain
- Engineers chain
- Revenue chain
1. Metric Chain:
- Metric chains are made in lengths 20m and 30m. Tallies are fixed at every five-meter length and brass rings are provided at every meter length except where tallies are attached.
2. Gunter’s Chain
- Length = 66’ (22 yards), No of links = 100, Each link = .66’
- Used for measuring distances in miles or furlongs (220 yards), acres (Area).
3. Engineer’s Chain
- Length = 100’, No of links = 100, Each link = 1’
- Used in all Engineering Surveys.
4. Revenue Chain
- Length = 33’, No of links = 16
- Commonly used for measuring fields in cadastral Survey.
Types of Measuring Tapes in Surveying
Tapes are made of different materials:
1. Cloth or linen tape
- Used for subsidiary measurements
- Very light, easy to handle
- May effect by moisture
2. Metric steel tape
- Made of steel
- Outer end is provided with a ring for holding
3. Invar tape
- Used for high precision work
- Made of alloy steel
4. Synthetic tape
- Made of glass fiber with PVC coating
- These are used for short measurements
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.