To erect a perpendicular to a chain line from a point on itThe method of establishing perpendiculars with the tape is base on familiar geometric construction. The flowing is same of the methods most commonly used.
Select E and F Equidistant from C hold the zero end on the tape at E and 10 meter end at F. pickup 5m mark, stretch the tight and establish D. joint DC .
Select any point F outside the chain line preferably at 5m distance from C. hold 5m mark at F 20m mark at C Witch as center draw and arg to cut the line at E. join EF and produce in to D such that EF = FD = 5m. next dust point D will lie at the 10m mark of the tape head along EF with end zero at E join DC.