Linear Methods of Surveying
It can be classified into three types:-
- Direct method
- Electronic method
- Optical method
The direct method is further divided into five types:-
- The pacing technique is mostly employed in preliminary surveys and explorations.
- In this method, the surveyor is required to conduct a rough survey quickly.
- It also roughly checks the distance measured by other means.
- The number of paces between the two points of the line is counted.
- Knowing the average length of the pace helps to know the length of the line.
- The length of the pace is dependent on the person who is measuring, the ground, the speed of pacing and the slope of the country.
- By walking on an approximate level ground over various lines of known lengths helps in determining the length of one's natural step.
- It is a difficult process to perform pacing over a rough surface or on slopes.
- The passometer is a watch-like instrument that is carried on the person's pocket or tied on the man's leg.
- This instrument records the paces when the man moves from one point to another.
- The distance is calculated by multiplying the number of paces by the average length of the pace.
- This method overcomes the monotony and strain in counting the paces by the surveyor.
- The pedometer is a device similar to passometer but it is adjusted to the length of the pace of the person carrying it.
- It registers the total distance covered by any number of paces.
- It automatically measures the distance.
- It is best in measuring vertical distances.
- The odometer is an instrument that measures the number of revolutions of a wheel.
- It is fitted to the wheel that is rolled along the line whose length is required.
- The distance covered is calculated by multiplying the number of revolutions taken to cover the length by the circumference of the wheel.
- This instrument does not provide accurate results on an undulating surface.
- When the surface is smooth, the speedometer used in vehicles can be used.
- In Chaining method, the distance is determined by means of a tape or a chain.
- This is one of the accurate methods to determine the linear measurements.
- The chain is used for ordinary precision.
- Tape or a special bar is used to measure distance with high precision.
- The distances determined by chaining form the basis of all surveying.
- It is the accurate and commonly employed method in surveying.
- No matter how accurately the angles may be measured, the survey can be no more precise than the chaining.
- The instruments used for determination of the length of a line by chaining are chain or tape, arrows, pegs, ranging rods, offset rods and plumb bob.
- Depending upon the material used, the tapes are classified as cloth or linen tape, metallic tape, steel tape and invar tape.
- Electronic equipment used for measuring distances is called an electronic distance measurement (EDM) instrument.
- They rely on the measurement of electromagnetic waves and measuring even fraction of wavelength by phase difference technique.
- Distances are measured with instruments that rely on propagation, reflection and subsequent reception of either radio or light waves.
- The various instruments that are used under the electronic methods are Geodimeter, Tellurometer, the Decca navigator and the Lambda Position Fixing System.
- The method of measurement in the case of Geodimeter is assessed on the propagation of modulated light waves.
- The other three instruments use radio waves for distance measurements.
- These instruments even display the distance measured.
- They are costly and delicate.
- For ordinary surveying, they are not used commonly.
- In this method, observations are taken through a telescope and distances are determined by calculation such as in tachometer or triangulation.
- The tachometer (stadia is the common term in the USA) is a surveying method utilized to quickly determine the horizontal distance to, and elevation of, a point.
- Stadia observations are accessed by sighting through a telescope equipped with two or more horizontal cross wires in a spacing that is known.
- The apparent intercepted length between the bottom and top wires is read onto a graduated rod held vertically in the desired stage.
- The space from the telescope to a rod is seen by proportional relationships in similar triangles.
*Source- Internet, Books, Self-Analysis