To measure distance between two points using direct ranging

To measure distance between two points using direct ranging

Objective:

The main objective of this job is to measure the distance between two points using direct ranging.

Apparatus:

  • Chain
  • Arrows
  • Fiber glass tapes
  • Ranging Rods
  • Plumb bob
  • Mallet
  • Wooden pegs

Least count of the instrument:

Least count of metric chain = 0.02 m

Least count of fiber glass tape = 0.01 m

Related theory:

Ranging a line

The process of establishing various intermediate points on a straight survey line joining two stations so that the distance between them can be accurately measured is called ranging.

When the survey line is short and the endpoints are clearly visible them chain or tape can be utilized to measure their length but, when a line is long or end stations are not clearly visible due to undulations on the ground surface then it is necessary to mark several points with ranging rods along the survey line to correctly determine their distance.

Ranging can be performed by direct observation by the naked eye or line ranger or by theodolite but in general theodolite is only employed for big-budget projects that require a high level of accuracy.

Types of ranging

Ranging can be classified as:

  1. Direct ranging
  2. Indirect or reciprocal ranging

Direct ranging

When intermediate ranging rods are fixed along the straight line, by direct observation from end stations, then the process is termed as direct ranging. Direct ranging is only possible when the end stations are intervisible.

Indirect ranging

When the end stations are not intervisible due to undulating ground, then the ranging rods are indirectly fixed along the survey line, and this is called indirect or reciprocal ranging

Ranging rods

Ranging rods are used for marking the position and location of the station points and ranging lines to make them clearly visible from even far away distance.

They are painted alternately black and white, or red and white or red white and black stripes to enhance their visibility. Two colored stripes also make it easier to differentiate the ranging rod from background objects.

A ranging rod is better for the location of a point than a peg because while using the peg we have to move from the center of one peg to the center of another peg, which can give some error in measurement owing to its bigger size but a ranging rod being thin, gives relatively accurate value.

a)Procedure for direct ranging

Following steps are adopted in performing in direct ranging:

  • Stand ranging rods vertically behind each end of the line.
  • Stand about 2 m behind the ranging rod from the beginning of the line.
  • Instruct the surveyor to hold a ranging rod vertically at an arm’s length at the point where the intermediate station is to be marked.
  • Direct the surveyor to move with the ranging rod in the right or left direction, until the three ranging rods exactly coincide in a straight line.
  • Bend down to sight and check the position of the rod by sighting its lower ends to remove errors due to the non-verticality of the ranging rods.
  • After ensuring that the three ranging rods are in a straight line, give the surveyor signal to fix the ranging rod at that point.

Code of signals

The following codes of signals are used in directing the surveyor in ranging a line:

Sr. # Signal Interpretation
1 Rapid sweeps with the right hand Move considerably to the right
2 Rapid sweeps with the left hand Move considerably to the left
3 Slow sweeps with the right hand Move slowly to the right
4 Slow sweeps with the left hand Move slowly to the left
5 Right arm extended Continue to move to the right
6 Left-arm extended Continue to move to the left
7 Right arm up and moved to the right Plump the rod to the right
8 Left-arm up and moved to the left Plump the rod to the left
9 Both arms above the head and then brought down Correct the ranging rod
10 Both arms extended forward and depressed rapidly Fix the ranging rod

b)Procedure for ranging and chaining:

  • Identify the endpoints of the survey line whose length is required to be measured using the direct ranging technique.
  • Fix arrows at the endpoints and hold ranging rods at these ends.
  • Unfold the chain from the starting point. Hold another ranging rod in between the two points at distance less than chain length.
  • Direct the third ranging rod to move to a position such that the ranging roads are exactly in a straight line.
  • Align the chain to pass through the ranging rod at the starting point and intermediate ranging rod. Fix one arrow at the end of the chain.
  • Repeat the procedure to get intermediate points.
  • Count the number of chain lengths and also the length of chain handles, to get the total length of the survey line.
  • Repeat measurement in the reverse direction.

Observations & Calculations:

The real length of chain =

No. of chain lengths measured =

The distance measured between points through direct ranging =

Results & Discussions:

The distance between two points using direct ranging has been measured successfully.

Precautions:

  • Personal protective equipment such closed toe shoes, helmets and safety vets, etc. should be worn in the field.
  • Follow the proper step-by-step procedure to avoid any errors.
  • Keep ranging rods exactly vertical by holding them with both hands.
  • Place the ranging rods on clean and leveled ground.
  • The station should be marked with wooden pegs.
  • Every station should be positioned with respect to three permanent points, if possible.
  • Avoid trespassing personal properties and crossing busy roads.
  • Establish station on firm ground.
  • Run survey line parallel to the leveled ground.
  • The signals should be made clear without any confusion.
  • When the surveyor is at a great distance, the instructor should use his handkerchief for signaling.
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