Indirect or Reciprocal ranging is done when the ends of survey line are not inter visible due to high ground or a hill intervening. It also done when the ends of survey line are not clearly 1visible due to long distance between them Let ‘A’ and ‘B’ be two survey stations. They are not mutually visible due to high ground between them. It is required to measure the distance between ‘A’ and ‘B’ Let two chainman at ‘M1’ & ‘N1’ are as shown in figure The chainman at ‘M1’ can see the both the ranging rods at‘N1’ and ‘B’ The chainman at ‘N1’ can see the ranging rods at ‘M1’ and ‘A’ . The two chainman then directs each other alternately. The chainman at ‘N1’ directs the chainman at ‘M1’ to come in line with ‘AN1’ to occupy position ‘M2’ Then the chain man at ‘M2’ directs the chainman at ‘N1’ to come in line with ‘M2B’ By successively directing each other, they finally occupy position ‘M’ and ‘N’ on line ‘AB’ Now ‘AMNB’ are exactly in the same line
Step wise procedure indirect ranging i.e. reciprocal ranging- Let A and B be the two station with rising ground or a hill intervening between them even i)Select any two intermediate station say M1 and N1 , between A and B with ranging rods such that from M1 points N1 and B are visible and from N1 , points M1 and A are visible ( as shown in fig.) ii) Chainman at M1 direct the chainman at N1 to in line with M1 B point ie position at N2 then chainman at N2 directs the chainman at M1 two in line with N2 to A ie position at M2 iii)In the next step Chainman at M2 direct the chainman at N2 to move in line with M2 B ie position at N3. iv) Thus by successively directing each other into line there position will be changed until finally they are both on the line A B exactly.