- 2076 Chaitra · 2+2 marks
Selecting main stations; number of offsets
Short answer
Main stations should be intervisible, form well-conditioned triangles, and allow short offsets with minimum lines on level ground, using a long base and check lines. The number of offsets depends on object shape: two for straight edges, many at close intervals for curves, and more at direction changes for irregular boundaries.
What are the major considerations while selecting the main station and lines of chain survey? How would you decide the number of offsets based on the objects?
Major considerations in selecting main stations and lines
- Stations must be intervisible and permanent, on firm ground.
- The framework should consist of well-conditioned triangles (angles between and ).
- The main (base) line should be the longest, on level ground, near the middle of the area.
- Use the minimum number of stations and lines.
- Lines should be close to the details so that offsets are short (preferably up to 15 m).
- Avoid obstacles and steep slopes; do not let lines pass over ponds or buildings.
- Provide check lines and tie lines for testing and for the details.
- Some lines may run along fences or roads to reduce the number of offsets.
Number of offsets depending on the object
The number of offsets depends on the nature of the object and the scale of the plan.
| Object | Offsets required |
|---|---|
| Straight boundary or wall | Two (one at each end), plus a check offset |
| Building (rectangular) | Offsets to each corner, or two at one side and a tie |
| Curved boundary (road edge, river bank) | Many, at short regular intervals (5 to 10 m), more for sharper curves |
| Irregular boundary | More offsets at every change in direction |
| Isolated objects (tree, pole, well) | One offset and the chainage |
| Open land of low importance | Few offsets, at 20 to 30 m |
The more irregular the shape and the larger the scale, the more offsets are needed, so that the drawn outline resembles the real shape.
✓ Answer checked by Claude Opus 5.5 (high effort)
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