Understanding Contouring Methods in Surveying

Contouring is the process of tracing imaginary lines on the ground that connect points of equal elevation. These lines, called contours, are then shown on a map to represent the terrain relief. Different methods are used for contouring depending on the terrain, required accuracy, scale of the map, and available time and resources.

Analyzing Contouring Methods for Hilly Terrain

When dealing with difficult or hilly terrain, certain contouring methods become more practical and efficient than others. The question specifically asks about a method effective in hilly terrain using radial lines.

• Vertical control method: This refers to the process of establishing points of known elevation (benchmarks) across the survey area. While crucial for any contouring project as it provides the elevation framework, it is not a method for collecting the detailed points needed to draw contours across the entire area, especially using radial lines from a single setup.
• Horizontal control method: Similar to vertical control, this method establishes points with known horizontal positions (coordinates). It provides the positional framework for the survey but doesn't describe the specific technique for collecting elevation data for contouring using radial lines.
• Method of cross-section: This method is typically used for linear projects like roads, railways, or canals. It involves taking levels along lines perpendicular to the center line at regular intervals. While it produces contours for a strip of land, it's not a general method for contouring an entire area, particularly one involving setting up at stations and taking readings along radial lines in hilly terrain.
• Tacheometric method: This method uses a tacheometer (a type of theodolite fitted with a stadia diaphragm) to quickly determine both the horizontal distance and the elevation difference of points from a single instrument setup. The observer sets up the instrument at a known point, takes readings to points whose contours are to be determined, typically along radial lines radiating from the instrument station. This method is rapid and well-suited for surveys in broken ground, rough terrain, or hilly areas where direct measurement of distances using chain or tape is difficult or time-consuming. By taking readings to numerous points along radial lines from strategic locations, sufficient data points can be collected to draw contours effectively.

Why Tacheometric Method is Suitable for Hilly Terrain with Radial Lines

The Tacheometric method is particularly effective for contouring in hilly areas using radial lines because:

• It allows for rapid collection of distance and elevation data for many points from a single station.
• It eliminates the need for chaining or taping distances on sloping or rough ground, which is difficult and prone to error in hilly terrain.
• Points can be sighted and their data recorded along radial lines from the instrument, covering a significant area around the setup point efficiently.
• This makes it possible to survey a large area with fewer instrument setups compared to methods requiring direct measurement or a dense grid of points.

Therefore, the Tacheometric method directly aligns with the description of contouring in hilly terrain by setting radial lines.

Conclusion

Based on the characteristics and typical applications of the contouring methods, the Tacheometric method is the one specifically designed for efficiently collecting surveying data, including elevations and distances along radial lines, making it highly effective for contouring in challenging terrains like hilly areas.

The final answer is the Tacheometric method.

Revision Table: Key Contouring Techniques

Reviewing the different methods helps solidify understanding:

• Tacheometric Contouring: Fast method using tacheometer, ideal for rough/hilly ground, surveys points along radial lines.
• Grid Method: Laying out a grid and taking levels at intersections; good for plain areas but slow in hilly terrain.
• Spot Levelling: Taking levels at salient points (summits, depressions, changes in slope) and interpolating contours; useful for small areas.
• Cross-Section Method: Specific to linear works, taking levels perpendicular to a centerline.

Additional Information on Tacheometric Surveying

Tacheometry is an indirect method of determining distances and elevations. It relies on optical principles using an instrument like a tacheometer or a total station. In stadia tacheometry, the tacheometer has stadia hairs in the diaphragm. By observing the staff intercept between the stadia hairs and the vertical angle to the point, the horizontal distance and vertical height can be calculated using specific formulas. This allows for rapid surveying of points without the need for separate distance measurements, which is a major advantage in uneven or difficult terrain.