The document describes a reciprocal method for measuring an obscured survey line AB using repeated alignment between two surveyors. The surveyor and assistant place poles at points C1 and D1 where they can see each other and the other poles. They then take turns positioning new poles along the lines until their poles C and D lie on the original line AB, allowing them to determine the measurement of the obscured portion of the line.

4. The reciprocal method of repeated alignment can be used to measure a survey line AB where
line AB is obscured by a rise in the ground. The surveyor and his assistant place themselves
with poles at C1 and D1 so that each can see the other three poles in addition to their own.
Assuming the surveyor to be at C1 he directs the assistant to position D2 on the line C1B. The
assistant then ranges the surveyor to C2 on the line D2A, the procedure being repeated until
the two poles C and D lie on the line AB.
C
A Section D B
C1 
 D1
 D2
C2 
D3 
C3 D4