--- title: "Field and office work; length of offsets" subject: "Surveying I" chapter: 3 key: "semester_3/surveying_i:chain-field-office-work-offset-length" source: https://www.bctengineer.com/q/semester_3/surveying_i/chain-field-office-work-offset-length site: BCT Engineering --- # Field and office work; length of offsets Surveying I · Chapter 3: Chain Survey - Asked in: 2080 Baisakh, 2+2 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/surveying_i/chain-field-office-work-offset-length - Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-3 ## Question Describe the field and office work in chain surveying. Explain the factors governing the length of offsets. ## Short answer Field work: reconnaissance, station marking, chaining, taking offsets, booking, check lines. Office work: checking, choosing scale, plotting triangle framework and offsets, inking, area computation. Offset length depends on scale, accuracy, method of setting perpendicular, nature of ground, and importance of the detail. ## Answer #### Field work 1. Reconnaissance of the area and preparation of an index sketch. 2. Selection and marking of stations and lines so that triangles are well-conditioned. 3. Ranging and chaining of the lines. 4. Taking and measuring offsets (perpendicular and oblique) to all details. 5. Booking of all measurements in the field book. 6. Check lines and ties measured to test accuracy. #### Office work 1. Check the field book, compute any corrections. 2. Choose a suitable scale and arrange the layout on the drawing sheet. 3. Plot the base line first; then plot triangles by intersecting arcs using the compass and scale. 4. Plot the offsets, and draw the details (buildings, roads, boundaries). 5. Check by scaling the check lines and compare with the field values. 6. Ink and colour the plan, add the title, north line, scale and a legend. 7. Compute the areas (triangles, offsets by trapezoidal or Simpson's rule). #### Factors governing the length of offsets 1. **Scale of the plan.** The error in plotting is at best 0.25 mm; at a smaller scale, a longer offset can be accepted. 2. **Accuracy required.** High accuracy needs short offsets. 3. **Method of setting the perpendicular.** A tape can give a good perpendicular up to about 15 m, a cross-staff up to 15 to 20 m, an optical square up to 20 m. 4. **Nature of the ground and details.** Offsets to well-defined permanent details must be short; rough details allow longer offsets. 5. **Obstacles and the ground.** Offsets over broken or undulating ground are made shorter. 6. **Importance of the object.** Important features are surveyed by short offsets or ties. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In office work, what is plotted first? A. The offsets and details B. The title and north line C. The base line, then triangles by intersecting arcs D. The area computations Answer: C. The framework is plotted before details. (Key point: Plotting order) 2. How is the plotted framework checked? A. Re-plot with a different scale B. Measure the north line C. Count the number of offsets D. Scale the check lines and compare them with the field values Answer: D. A difference between scaled and measured check line shows plotting or field error. (Key point: Check in the office) 3. How does the scale of the plan affect the allowable offset length? A. At a smaller scale a longer offset is acceptable B. At a smaller scale offsets must be shorter C. Scale has no effect D. Only the larger scale permits long offsets Answer: A. Plotting error is about 0.25 mm; at small scale that is a large ground distance, so errors in long offsets don't show. (Key point: Scale and offset length) 4. Which factor shortens offsets? A. Plans of low accuracy B. High accuracy requirement or broken ground C. Smooth level ground with rough details D. Use of a smaller scale Answer: B. Important, accurate work and undulating ground need short offsets. 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