Q7. Which of the following is a disadvantage of plane table survey?
Correct Answer: B. It is essentially a tropical instrument.
📚 Detailed Explanation: Disadvantages of Plane Table Surveying
Plane table surveying is a graphical field method where map plotting is done simultaneously with observation. To identify the correct answer, you must distinguish genuine disadvantages from advantages that have been reworded.
Why “essentially a tropical instrument” is a disadvantage:
The plane table uses a drawing sheet stretched across an open board, with plotting done in the open air. It cannot be used in wet, rainy, or windy weather because rain distorts and damages the paper and wind makes accurate sighting and drawing impossible. Dense forests also obstruct sighting. This makes it suitable only in dry, open, tropical or semi-arid climates — a significant limitation for surveys in temperate or wet regions.
The plane table uses a drawing sheet stretched across an open board, with plotting done in the open air. It cannot be used in wet, rainy, or windy weather because rain distorts and damages the paper and wind makes accurate sighting and drawing impossible. Dense forests also obstruct sighting. This makes it suitable only in dry, open, tropical or semi-arid climates — a significant limitation for surveys in temperate or wet regions.
Why the Other Options Are Advantages, Not Disadvantages
| Option | Correct classification | Explanation |
|---|---|---|
| A — Replaces compass in magnetic areas | Advantage | A plane table uses geometric sighting rather than a magnetic needle. It is unaffected by local magnetic attraction, making it superior to a compass in areas with magnetic ore bodies or iron structures. |
| C — Suitable for small-scale maps | Advantage | The plane table excels at rapid small-scale and medium-scale mapping because the map is built graphically in the field, reducing office work. |
| D — Simple and cheaper than theodolite | Advantage | The equipment (table, alidade, drawing sheet) is simpler, lighter, and less expensive than a theodolite-and-total-station setup. This is an advantage, not a disadvantage. |
Complete Disadvantages of Plane Table Survey (for revision)
| # | Disadvantage |
|---|---|
| 1 | Essentially a tropical instrument — unsuitable in rain, strong wind, or cold humid conditions |
| 2 | Heavy and cumbersome to transport (large board, tripod, accessories) |
| 3 | Less accurate than theodolite or total station survey |
| 4 | Paper distorts with changes in humidity and temperature, affecting plotted accuracy |
| 5 | Requires skilled draughtsmen for accurate field plotting |
| 6 | Cannot be used for accurate control surveys over large areas |
Key Concepts for Students
- Options A, C, D are advantages in disguise: This is a classic exam trap — three of the options describe genuine strengths of plane table surveying. Carefully read every option before selecting, and verify that the chosen option is a limitation, not a benefit.
- “Tropical instrument” is the standard textbook phrasing: This exact phrase appears in all standard surveying textbooks (Bannister, Schofield, Punmia) as the primary disadvantage of plane table surveying. Memorise it as-is for objective questions.
- Weather limitation is practical: In India and similar tropical countries, plane table surveying is widely used because of the predominantly dry weather. In the UK, Scandinavia, or monsoon seasons, the method becomes impractical, illustrating why the “tropical instrument” label is a genuine disadvantage in a global context.