Q18. When compared with Chain Surveying, Plane table is:
📚 Detailed Explanation: Accuracy of Plane Table vs Chain Surveying
Both plane table and chain surveying are field measurement techniques, but they differ significantly in the precision achievable because their measurement tools and methods are fundamentally different.
Plane table surveying is a graphical method. Distances and angles are represented directly on a drawing sheet, and accuracy is limited by:
• The scale of the drawing (typically 1:500 to 1:5000)
• Plotting precision (the human hand and a pencil point limit accuracy to about 0.2–0.5 mm on the sheet)
• Paper distortion from temperature and humidity
• Orientation errors at each station
Chain surveying, by contrast, uses a steel chain/tape for linear measurement directly on the ground, achieving accuracy in the range of 1:1000 to 1:3000 of the measured distance or better.
Accuracy Comparison
| Feature | Chain surveying | Plane table surveying |
|---|---|---|
| Measurement type | Direct linear (chain/tape) | Graphical (drawn on sheet) |
| Typical accuracy | 1:1000 to 1:3000 | Limited by plotting scale & paper distortion |
| Angular control | Geometric triangulation of triangles | Visual ray direction through alidade |
| Error sources | Chain sag, tape tension, slope | Plotting error, orientation error, paper shrinkage |
Why Options B, C, D Are Wrong
B — Not accurate: This is too strong a statement. Plane table surveying is acceptably accurate for small-scale topographic maps and reconnaissance surveys. It is “less accurate” than chain surveying, not completely inaccurate.
C — More accurate: Chain surveying uses direct physical measurement on the ground, which is inherently more accurate than graphical approximation on a scaled sheet. Plane table cannot match this.
D — Accurate (same as chain): The two methods are not equal in accuracy. The graphical nature of plane table limits it to a lower precision tier compared to direct linear measurement by chain.
Key Concepts for Students
- Graphical vs. numerical methods: Chain surveying records numbers (distances, bearings) that can be checked arithmetically. Plane table records geometry directly on paper, which cannot be checked mathematically once the map is drawn. Graphical methods are inherently less precise.
- Plane table is suited for reconnaissance and topographic work: Despite lower accuracy, plane table is faster and produces a visual map immediately. It is well suited for small-scale mapping, route reconnaissance, and terrain sketching where precision to the centimetre is not required.
- The word “less accurate” is relative: Option B says “not accurate,” which implies it is useless. Option A says “less accurate,” which correctly identifies a relative comparison. Always prefer the precise relative term over an absolute negative when comparing two functional survey methods.