Basic definitions in levelling: At a given point, the straight line normal to the plumb line (i.e., perpendicular to the direction of gravity) is known as which line?

Introduction / Context:
Levelling distinguishes between several reference lines and surfaces. Confusing these terms can lead to conceptual errors when staking elevations or interpreting instrument readings.

Given Data / Assumptions:

• Plumb line indicates the local direction of gravity (vertical line).
• We seek the straight line perpendicular to this direction at a point.
• Standard surveying definitions apply.

Concept / Approach:
A horizontal line is a straight line that is tangential to the level surface at a point and is perpendicular to the plumb line. A level line, by contrast, is a curved line lying on a level surface (everywhere normal to gravity) connecting points of equal potential; it is not straight over long distances. The datum line is a reference line or zero-elevation baseline, not defined by gravity at a point.

Step-by-Step Solution:

Verification / Alternative check:
Using a spirit level, the bubble centered ensures the line of sight is horizontal (perpendicular to the plumb line), matching the definition.

Why Other Options Are Wrong:

• Level line: represents a curved locus of points; not a straight line normal at a point.
• Datum line: arbitrary reference; unrelated to the local gravity direction.
• Vertical line: coincides with the plumb line, not its normal.

Common Pitfalls:
Using “level” and “horizontal” interchangeably; forgetting that the Earth's level surface is curved.

Final Answer:
Horizontal line