Sheet Junction Offset and a Doubtful Field-Book Entry
A fair sheet at 1:2000 is being compiled for a village in East Champaran. The field book gives the distance between traverse stations 17 and 18 as 62.40 m. On the fair sheet, the same line scales 61.9 m at the stated scale. A road crossing the sheet junction is offset by 2.5 mm from its position on the adjacent sheet. The paper has shrunk 0.8% since printing. A pencilled entry in the field book has been erased and overwritten as 62.40?, and the signature column is blank. The surveyor wants to shift the road 2.5 mm to make the sheets match and submit the sheet, because the contour interval is 0.5 m and the next survey camp is due. The sheet is to be used for land acquisition measurements.
1. Convert the discrepancies to ground distances and compare with paper shrinkage. Is the 2.5 mm road offset explained by shrinkage?
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Scale conversion
- 2.5 mm at 1:2000 = $2.5\times2000=5,000$ mm = 5.00 m on the ground.
- 61.9 m plotted versus 62.40 m booked: difference 0.50 m, which at 1:2000 is 0.25 mm on paper.
- 0.8% shrinkage on 62.40 m is $62.40\times0.008=0.499$ m, essentially the same 0.50 m. So the 61.9 m scaled length is consistent with the 62.40 m field length plotted on paper that has shrunk: $61.9/0.992=62.40$ m.
The 5.00 m road offset is not explained by shrinkage: 0.8% of 5.00 m is only 0.04 m. The line-length discrepancy is a paper-scale effect; the junction offset is a real plotting or control problem.
2. Diagnose the possible sources. Should the road be shifted or the field book rechecked? What about the erased entry?
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Diagnosis
The station 17-18 line should not be corrected again; once shrinkage is allowed for, it agrees. The road offset of 5 m is far too large for paper movement. Likely causes are plotting from the wrong traverse station, a sheet-junction coordinate mismatch, a field-book booking error, or the adjacent sheet using a different control or scale. Shifting the road 2.5 mm to make the sheets match would create a false alignment and would corrupt any land-acquisition measurement taken from the sheet.
The erased and overwritten entry with no signature is not acceptable evidence. Treat the line as doubtful until it is rechecked against the original field book, the traverse computation, and an independent measurement. Re-observe the line if the original cannot be validated.
3. What record and correction procedure should be followed before the sheet is submitted?
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Record procedure
- Do not erase or overwrite. Draw a single line through the doubtful entry, write the correct value only when supported, and add initials, date, and reason.
- Attach the original field-book page and the traverse computation to the fair sheet.
- Plot the sheet junction from common control coordinates, not by matching the road graphically; check at least two common points.
- If the 17-18 line is confirmed at 62.40 m, note that the fair-sheet scale check must be made on unshrunk paper or corrected by the measured shrinkage factor.
- Mark the road at the junction as to be verified and do not submit the sheet as final until the control mismatch is resolved. For land acquisition, an unresolved 5 m offset is material and must be reported.