Levelling and Contouring

Why it matters

Levelling and contouring are crucial in civil engineering for determining the elevation of points on the earth's surface. This information is vital for designing infrastructure, ensuring proper drainage, and planning construction projects.

Key ideas

• Levelling: The process of determining the height of one level relative to another. It is used to establish the elevation of points or to transfer a known elevation from one point to another.
  • Types of Levelling: Differential levelling, profile levelling, and reciprocal levelling.
  • Instruments: Dumpy level, auto level, and digital level.
• Contouring: The process of connecting points of equal elevation on a map to represent the terrain.
  • Contour Interval: The vertical distance between two consecutive contour lines.
  • Characteristics of Contours: For ordinary single-valued terrain, contours do not cross, close on themselves (possibly beyond the map), and closer spacing indicates steeper ground. Vertical or overhanging cliffs require special interpretation.

Formulas

• Height of instrument (line-of-collimation elevation): HI = RL_BM + BS.
• Reduced level of the foresight point: RL_next = HI − FS = RL_BM + BS − FS.
• For a completed levelling run, ΣBS − ΣFS = last RL − first RL. This arithmetic check does not detect every observational error.

Worked example

Benchmark RL = 100 m, backsight = 1.5 m, foresight = 0.5 m from the same setup. HI = 100+1.5 = 101.5 m. RL of the foresight point = 101.5−0.5 = 101.0 m. The latter is the point elevation, not HI. Balance backsight and foresight distances where practicable to reduce collimation and curvature/refraction effects.

Common mistakes

• Confusing backsight and foresight readings.
• Incorrectly calculating the contour interval.
• Misinterpreting contour lines on a map.

For GATE CE

Questions often involve calculating elevations using levelling data or interpreting contour maps. Practice problems on differential levelling and contour characteristics.

Quick check

1. What is the purpose of levelling in surveying?
2. How do contour lines indicate a steep slope?
3. What is a backsight reading?

Answers: 1. To determine the elevation of points. 2. Contour lines are close together. 3. A reading taken on a level staff held at a point of known elevation.