Basic concept in surveying: The operation of determining the difference in elevation (RL difference) between two points on the Earth’s surface is called what?

Introduction / Context:
Surveying distinguishes among several levelling procedures, but all serve the core purpose of finding elevation differences. Understanding the umbrella term and its specific variants helps select the right method for site work, profiles, and transfer of benchmarks.

Given Data / Assumptions:

• Two points need an elevation difference.
• Standard levelling instruments (automatic/dumpy level) and staff are used.
• Terminology per conventional surveying texts.

Concept / Approach:
Levelling is the general operation of determining elevations or differences in level. Within it, simple levelling uses a single set-up visible to both points; differential levelling uses a series of set-ups via change points to connect points far apart; longitudinal (profile) levelling determines elevations along a route for design sections. The question asks for the general name of the operation, hence “Levelling.”

Step-by-Step Solution:

Verification / Alternative check:
Standard definitions: “Levelling is the art of determining elevations of points or their differences of level.” Differential, simple, and longitudinal are subtypes.

Why Other Options Are Wrong:

• Simple, differential, and longitudinal levelling are specific methods. The stem is generic; therefore, the broad term “Levelling” is most accurate.

Common Pitfalls:
Assuming the question always intends “differential levelling”; overlooking visibility constraints that distinguish methods.

Final Answer:
Levelling (general process)