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Ranging across high ground

Short answer

Ranging is establishing intermediate points on a straight line between two end stations. When ends are non-intervisible due to high ground, reciprocal ranging is used: two assistants on the high ground alternately align each other onto the line between A and B, then intermediate points are set by direct ranging.

Define ranging. Explain how a survey line is ranged between two points that are not intervisible due to intervening high (raised) ground; explain the procedure with a neat sketch.

Ranging is the process of establishing intermediate points on a straight line between two end stations so that the chain or tape can be laid in a straight line along them. It is done with ranging rods (direct ranging) when the ends are intervisible, and by reciprocal ranging when they are not.

Ranging when the ends are not intervisible

When the points A and B are on opposite sides of a hill (high ground), a person at A cannot see B. In this case reciprocal ranging is used.

Procedure

  1. Fix ranging rods at A and B.
  2. Choose two points M1 and N1 on the high ground, close to the line AB, such that from M1 both N1 and B can be seen, and from N1 both M1 and A can be seen.
  3. Two assistants stand at M1 and N1. The assistant at M1 directs the one at N1 to move to N2 on the line M1B.
  4. Then the assistant at N2 directs M1 to move to M2 on the line N2A.
  5. The process is repeated (M2 directs N2 to N3 on the line M2B, and so on) until M, N, A and B all lie on one straight line.
  6. Once M and N are fixed, intermediate points between A and M, M and N, and N and B are fixed by direct ranging.
          M1 ........ N1
        /     hill     \ 
   A  /                  \  B
   (A, M, N, B finally in one line)

The line is then chained over the high ground in the usual way.

✓ Answer checked by Claude Opus 5.5 (high effort)

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