Field procedure of chain surveying

The field procedure of chain surveying follows these steps.

1. Reconnaissance

The surveyor walks over the area, observes its shape, details and obstacles, and plans the survey lines.

2. Selection and marking of stations

• Select main stations so that the main lines form well-conditioned triangles (angles $30^\circ$ to $120^\circ$).
• Select the base line (the longest, on level ground), the check lines and the tie lines.
• Mark stations with pegs or ranging rods, and tie them to permanent objects so they can be found again.

3. Index (key) sketch

A neat freehand sketch (not to scale) shows the whole area, the stations, the lines and the main details. Station names (A, B, C ...) are written on it.

        C
       /|\ 
      / | \ 
     /  |D \     A, B, C = stations
    A---+---B    D = check line CD

4. Ranging and chaining the lines

• Each line is ranged between its end stations with ranging rods so that the chain stays in a straight line.
• The leader and follower pull the chain. The follower stands at the starting station, and the leader carries 10 arrows, sticking one into the ground at the end of each chain length.
• The follower picks up each arrow as he reaches it; the number of arrows collected gives the number of whole chain lengths, and the last part length is read on the chain. All the main lines are chained in this way.

5. Taking offsets

• Perpendicular offsets are set with a cross-staff, optical square or the 3-4-5 tape method and measured with the tape.
• Oblique offsets or ties are used for far or hidden objects.

6. Booking

All chainages and offsets are written in the field book from the bottom of the page upwards, with the objects sketched. The left and right offsets are placed on either side of the chain line column.

7. Checking

• Measure check lines (for example, a line from the apex to a point on the opposite side) and tie lines; compare the measured length with the length scaled from the plot.
• Measure each line twice, forward and backward.

8. Office work

Plot the triangles to scale using arcs, plot the offsets, draw the details and ink the plan, then compute areas.

Chain surveying is best for small, open and fairly level areas.