Sliding arm is graduated and plumb line is attached to the fixed arm. It is based on similar triangles

Advantage: It can be made in the forest if no other instrument is available

In fig. AB= Tree, CD= Scale arm, CL=Fixed arm & PN= Sliding arm

CP in cm = CB in m.

In triangles ABC & MNL,

AB:BC = MN:LN:MN = CP (MN in cm = AB in m.)

AB = (BCxMN) / CP