Abstract: This invention relates to a method of drilling apertures for branch lines (A) blocked by renovation lining of a main pipeline (B) which comprises setting a water plug (1) in the opening of each branch line (A) prior to lining, said water plug (1) carrying a position marker (14) comprising an antenna coil and a resonator adapted to be excited by a driving signal and, after stoppage of the driving signal, release a resonance signal remaining in the resonator from the antenna coil and said position marker being situated in a position which, upon setting of the water plug (1) in position, is substantially concentric with the axis of the branch line (A), causing an antenna assembly for detecting the resonance signal to travel within the main pipeline (B) to detect the position of the marker (14) and driving a drilling means (24) using the thus-detected marker position as a target from within the main pipeline.