Survey instruments — practical application: In civil engineering fieldwork, a planimeter is specifically used for measuring which quantity on maps or plotted drawings? Provide the single best answer.

Introduction / Context:
A planimeter is a classic mechanical (and today, sometimes digital) instrument used in surveying, civil engineering, hydrology, and cartography. Its job is to measure areas directly on a plan or map without having to subdivide the figure into regular shapes. Understanding what the planimeter measures prevents misuse and saves time during quantity takeoff and design review.

Given Data / Assumptions:

• The instrument is a polar or rolling planimeter intended for 2D plan work.
• The operator traces the boundary of a closed figure on a scaled drawing.
• Map scale is known to convert plan area to ground area if required.

Concept / Approach:
A planimeter integrates the boundary path of a closed figure to compute its area. The internal mechanism (wheel and disc or digital encoder) performs an analog of line integral mathematics. Because the device responds to the path traced, it yields the area enclosed by the boundary, independent of the figure’s shape complexity, provided the tracing is closed and the instrument is properly zeroed and calibrated.

Step-by-Step Solution:

Verification / Alternative check:
Compare the planimeter result with a grid-count or coordinate (double-meridian distance) method on a simpler test polygon. Close agreement validates the instrument and technique before measuring complex areas like reservoirs or catchments.

Why Other Options Are Wrong:

• Contour gradient and slope angle: These are angular/gradient properties, not directly measured by a planimeter.
• Volume: Requires area * depth or integration in 3D; a planimeter alone measures 2D area on plans.
• None of these: Incorrect, because area is exactly what a planimeter measures.

Common Pitfalls:
Not closing the trace; ignoring the instrument constant; forgetting to square the scale ratio when converting plan units to ground units.

Final Answer:
Area