Select the INCORRECT obstacle in chaining in a chain survey:
📝 Detailed Explanation: Understanding Field Obstacles
Obstacles in chain surveying refer to physical barriers encountered in the field during the measurement process. These challenges are all about what you can see (ranging) and where you can physically stretch the chain (chaining).
Core Concept: Fieldwork vs. Office Work
"Plotting" is the task of drawing the map based on the field measurements. This is done in an office after the fieldwork is complete and is therefore not a physical obstacle encountered during the chaining process.
Categories of Physical Obstacles in Chaining
While it is always best to select survey stations to avoid obstacles, sometimes they are unavoidable. These field obstacles are grouped into three main categories:
1. Obstacles to Ranging (Vision Obstructed, Chaining Free):
This occurs when you can stretch the chain between two points, but cannot see from one end to the other to set out intermediate points in a straight line.
- Example: A small hill or high ground between two stations. The method of reciprocal ranging is used to overcome this.
- Example: Intervening trees and bushes.
2. Obstacles to Chaining (Chaining Obstructed, Vision Free):
This occurs when you can see the two points, but you cannot physically measure the distance between them directly with a chain.
- Example: A pond, lake, or river. Special geometric methods are used to calculate the distance across the obstacle without physically chaining it.
3. Obstacles to Both Ranging and Chaining (Vision and Chaining Obstructed):
This is the most challenging type of obstacle, where you can neither see nor chain across the feature.
- Example: A large building or dense woodland between two survey stations.