Principle of chain survey; types of offsets

Chain surveying is the type of surveying in which only linear measurements (with a chain or tape) are made in the field, without angular measurements. It is suitable for small, fairly flat areas with few details.

Principle

The area to be surveyed is covered by a network of well-conditioned triangles, because a triangle is the only simple figure that can be plotted from the lengths of its three sides alone. The sides (chain lines) are measured in the field. The details (buildings, roads, boundaries) are then located by measuring perpendicular or oblique offsets from the chain lines. On paper, the triangles are plotted to scale by intersecting arcs, and the details are plotted from the offsets.

        C
       /|\ 
      / | \       triangle ABC: sides measured
     /  |  \      tie line / check line shown
    A---+---B     offsets to details from AB, BC, CA

The work follows the principle of working from whole to part: the big triangles (framework) are made first and the details are surveyed from them. To avoid accumulation of errors, check lines are measured.

Offsets

An offset is the lateral measurement from a chain line to a point of detail.

• Perpendicular offset: measured at right angles to the chain line. The right angle is set with a cross-staff, optical square, or a tape (3-4-5 method). For a short offset the foot is found by swinging the tape and taking the shortest reading.
• Oblique offset: measured at any angle, when a perpendicular cannot be taken (obstacle, long distance). It is fixed by two measurements from two points on the chain line (a tie), or by an angle and a length.

   chain line  A----+------+------B
                    |      \ 
          perpendicular     \ oblique
                  object P    object Q

Both types are booked in the field book with the chainage along the line and the offset length on the left or right of the line. Perpendicular offsets are kept short (usually within 15 m) and are preferred for accuracy.