Perpendicular  Offsets  Using Cross Staffs

                              (a)                                          (b)                               (c)

.                                                            Cross staff

Above figure shows 3 different types of cross staffs used for setting perpendicular offsets. All cross staffs have two perpendicular lines of sights. Cross staffs are mounted on stand. First line of sight is set along the chain line and without disturbing setting right angle line of sight is checked to locate the object. With open cross staff (Fig.a) it is only possible to set perpendicular, whereas with french cross staff (Fig (b)), even 45º angle may be set. Since there are graduations and upper drum can be rotated over lower drum adjustable cross staff can be also used to set any angle.