Plane tabling instrument for direct (tacheometric) distance and elevation from the board In plane tabling, which instrument allows the surveyor to obtain horizontal and vertical distances directly—without resorting to chaining—by using stadia observations from the board?

Introduction / Context:
Plane tabling combines field observation and plotting. For distance without chaining, a stadia-equipped telescope is used so that distances and elevations can be derived from staff intercepts. This capability is integrated into a special alidade for plane table work, distinct from a separate theodolite-like tacheometer.

Given Data / Assumptions:

• The goal is to obtain distances/elevations directly from the board.
• Stadia hairs in the telescope permit tacheometric reductions.
• We distinguish between a simple (plane) alidade and a telescopic stadia alidade.

Concept / Approach:

The telescopic alidade used in plane tabling has a telescope with stadia hairs, enabling tacheometric measurements while plotting rays on the sheet. A simple plane alidade has only sight vanes and cannot provide stadia distances. A separate tacheometer is a different instrument; although it also uses stadia, it is not the plane-table attachment in question.

Step-by-Step Solution:

Verification / Alternative check:

Plane-table manuals describe the telescopic alidade as enabling “tacheo-platting”, confirming it provides horizontal and vertical distance data from stadia.

Why Other Options Are Wrong:

Plane alidade: no telescope, no stadia.

Clinometer: measures vertical angle only, not distance.

Tacheometer: not the plane-table attachment implied by the question.

Common Pitfalls:

Confusing tacheometer with telescopic alidade; forgetting to note the stadia constants for the specific alidade.

Final Answer:

Telescopic alidade (with stadia hairs)