Offset Instruments:

Offsets could be accurately set up, particularly long offsets, through use of optical instruments like cross staff or optical square etc.

Cross Staff

Two pairs of vertical slits, mounted orthogonally on a pole staff constitutes a simple instrument for setting out normal offsets as depicted in Figure(a). Every pair of slits could form a line of sight. The pole staff has adjustable length to remain the line of sight adjusted to surveyor's individual comfort.

Figure: Offset by Cross Staff

The normal to survey line AB at chain point C can be drawn by first adjusting the cross staff stationed at C such that one pair of vertical slits is aligned along line AB. A ranging rod D is after then placed approximately at right angle to line AB from C and is moved sideways till it is sighted along the other pair of slits in line with slit's hairline. In such a position, line CD will be perpendicular to chain line AB as shown in Figure(b).

If perpendicular is sought to be drawn on chain line AB from a fixed location D, then characterising some special ground feature, the cross staff is set at a point C on survey line AB such that CD is approximately perpendicular to AB. One line of sight created through one pair of vertical slits is then aligned along line AB. The cross staff is then moved sideways along line AB keeping one pair of slit aligned along AB and the other line of sight (i.e. second pair of vertical slits) brings D to coincide along with its hairlines. This situates the position of C on the line AB such that CD is perpendicular to AB.