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Factors of closing error; graphical adjustment

Short answer

Closing error arises from mistakes in length, bearings, local attraction, plotting, or booking. For graphical adjustment by Bowditch's rule, draw a base line representing the perimeter; at its end erect a perpendicular equal to the closing error. Ordinates from cumulative station points on the base to the line from start to top give corrections applied parallel to the closing direction.

List out the factors causing closing error of compass traverse. Explain the graphical adjustment of closing error of plotting of compass traverse.

Factors causing closing error in a compass traverse

  1. Errors in measured lengths (incorrect tape length, slope not reduced, sag, wrong tension, miscounted chain lengths).
  2. Errors in observed bearings (compass not centred or levelled, sticky needle, reading mistakes, parallax, wrong back-sight).
  3. Local attraction at a station that has not been detected or corrected.
  4. Errors in plotting (wrong scale, protractor error, thick pencil lines, shrinkage of paper).
  5. Booking and calculation mistakes (wrong FB/BB recorded, wrong conversion).

Graphical adjustment of the closing error (Bowditch's rule)

The closing error is distributed so that the correction at a station is proportional to its cumulative distance from the starting point.

  1. Plot the traverse A-B-C-D-E-A′; the closing error is the line A′A.
  2. Draw a base line equal to the perimeter and mark the points b, c, d, e at the distances AB, AB+BC, AB+BC+CD, ... from A; the last point is A′.
  3. At A′ draw a perpendicular A′X equal to the closing error (plan scale) and join AX.
  4. Vertical ordinates from b, c, d, e to AX are the corrections for B, C, D, E.
  5. On the plan, from each of B, C, D, E, draw a line parallel to A′A and set off the correction towards A. Join the new points to give the adjusted traverse.
                                      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)

✓ Answer checked by Claude Opus 5.5 (high effort)

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