at the paper machine by means of the marker agent injection system 35 39 into a dilution water feed pipe 33 located at a cross direction point x 0 above the web as a result the paper web w is marked with a marker line m forming a kind of cross direction 34 reference 34 line at the marker agent injection point x 0 as the web w undergoes a possible cross direction 34 drift 34 and shrinkage due to reasons described above on reaching the measurement beam 40 the marker line m on the dried web wd has shifted by a cross direction distance u03b4x relative to the initial injection point x 0 of the marker line m it must be noted herein that the web path in fig1 between the headbox 10 and the measurement beam 40 includes such sections as the paper machine former dryer press and a possible finishing section e g a sizing press and or a machine glaze calender all of which are not shown and that the measurement beam 40 is positioned just prior to the reel up section not shown referring to fig1 and 2 the measurement beam 40 is adapted to carry a measurement apparatus or carriage 42 capable of detecting the cross direction shift u03b4x of the marker line m this measurement apparatus may be formed by e g a set of radiation sensors 43 1 43 m that sensor 43 r of the set which coincides with the marker line m receives the maximum intensity of radiation r and issues the corresponding position.