Checking open traverse; Bowditch graphically

Checking an open traverse in compass survey

An open traverse has no closing check, so extra observations are taken:

1. Fore and back bearings of every line are observed; FB − BB should be $180^\circ$ (detects local attraction).
2. Cross-checks to a distant landmark: a prominent object is sighted from several stations; plotted rays must meet at one point.
3. Check lines (tie lines) are measured between non-adjacent stations and compared with the plotted distance.
4. Linking to known points: the traverse is closed on a station of known position or bearing (link traverse) and the closure is checked.
5. Repeat measurement of lengths (forward and back).

Graphical Bowditch adjustment of a closed traverse

Bowditch's rule distributes the closing error in proportion to the lengths of sides, so the correction at a station is proportional to the distance travelled from the start.

1. Plot the traverse A-B-C-D-E-A′. The closing error is A′A.
2. Draw a base line AA′ equal to the perimeter and mark b, c, d, e at the cumulative distances AB, AB+BC, ...
3. Erect A′X perpendicular at A′ equal to the closing error and join AX.
4. Ordinates from b, c, d, e to AX give the corrections of B, C, D, E.
5. On the plan, draw through each station a line parallel to A′A and mark off these corrections; join the shifted points.

                                      X
                                    . |
                                .     |
                          . e'        |
                    . d'  |           |
              . c'  |     |           |
        . b'  |     |     |           |
 A -----b-----c-----d-----e-----------A'
 (base = perimeter, marks at cumulative distances;
  A'X = closing error, drawn at right angles)

The corrected traverse closes at A.