Chain survey principle and operations

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.

Operations involved in chain surveying (field procedure)

1. Reconnaissance. The area is visited and inspected, and the plan of the survey is decided.
2. Selection and marking of stations. Main stations are chosen so that lines form well-conditioned triangles; stations are marked with pegs and ranging rods.
3. Index (key) sketch. A neat freehand sketch of the area is drawn showing stations, lines and main objects.
4. Ranging and chaining. Each line is ranged between its end stations, and chained from the starting station.
5. Taking offsets. Perpendicular or oblique offsets to the details are measured with the tape and cross-staff/optical square.
6. Booking. All chainages and offsets are recorded in the field book, working from bottom to top of the page.
7. Checking. Check lines and tie lines are measured to test the accuracy of the framework.

 Recon -> Stations -> Key plan -> Chaining
      -> Offsets -> Booking -> Checks -> Office

The measurements are then plotted in the office.