Types of offsets and how measured

An offset is a lateral measurement taken from a chain line to a point of detail. There are two types.

1. Perpendicular offsets

Measured at right angles to the chain line.

• Setting the right angle:
  • With a cross-staff: the surveyor stands on the line, sights along the line through one pair of slits, and directs the assistant to the object along the perpendicular pair of slits.
  • With an optical square (prism square): two mirrors at $45^\circ$ show the ranging rod on the line and the object in one image when the surveyor is at the foot of the perpendicular.
  • With a tape (3-4-5 method): a triangle with sides 3, 4 and 5 m (or multiples) gives a right angle.
  • For short offsets, swing the tape: hold the zero at the object and swing the tape across the line; the shortest reading on the chain line is the foot of the perpendicular.
• Taken for details close to the line, usually within 15 m.

2. Oblique offsets (ties)

Measured at any angle, when the object is far away or a perpendicular is difficult to set.

• Two-distance method: the object $P$ is joined to two points $A$ and $B$ on the chain line, one on each side of the foot of its perpendicular, and the distances $AP$ and $BP$ are measured. $P$ is plotted by intersecting arcs.
• Angle and distance: the angle at a point on the chain line and the length to the object are measured.
• Used for corners of buildings, field corners and objects that are hard to reach by a perpendicular.

 chain line ----A-------------B---->
                 \           /
              AP  \         /  BP     oblique offsets (ties)
                   \       /
                    \     /
                       P

Booking

Offsets are booked in the field book at the chainage where they are taken. For example: a perpendicular offset of 8 m to the right (to a tree) at chainage 45, and oblique offsets of 12 m and 14 m to a building corner P1 on the left, taken from chainages 50 and 60.

The chainage runs from the bottom of the page upwards, and each offset is written beside the chain line on the side where it lies, with the object's name or a sketch.