When two points of surveying are mutually invisible, which of the following methods of ranging is adopted?

🎯 Understanding the Concept of Ranging

Ranging is the fundamental process of establishing intermediate points along a straight line between two endpoints. This is essential in chain surveying to ensure that all measurements are taken along the correct alignment.

🔬 Direct vs. Indirect Ranging

The method used for ranging depends entirely on whether the two endpoints are visible to each other.

• Direct Ranging: This method is used when the two survey stations (e.g., A and B) are mutually visible. A surveyor at station A can sight station B and direct an assistant to place ranging rods at intermediate points directly on the line of sight. It is the simpler and more common method.
• Indirect Ranging (or Reciprocal Ranging): This method is necessary when an obstacle, such as a hill, a large building, or a dense forest, prevents the two endpoints from being visible to each other. Since a direct line of sight is impossible, a special procedure is required to establish the straight line "through" the obstruction.

Therefore, when points are mutually invisible, indirect ranging is the adopted method.

🗺️ Visualizing Indirect Ranging

The process of indirect ranging involves selecting two intermediate points (C and D) that are close to the actual line and are positioned so that C can see D and B, and D can see C and A. Through a series of successive approximations, C and D are moved sideways until they fall perfectly on the straight line AB.

💡 Key Takeaway

The choice between direct and indirect ranging is dictated by one simple factor: intervisibility. If the endpoints can see each other, use direct ranging. If they can't, indirect ranging is the required technique.