Plane table alignment – effect of cross-tilt on parallelism of line of sight and fiducial edge If the plane table is not horizontal in the direction at right angles to the alidade, the line of sight will be parallel to the fiducial edge only for which type of sight?

Introduction / Context:
With the plane table, the alidade's fiducial edge is intended to represent the line of sight when the board is correctly leveled. However, cross-tilt (tilt at right angles to the alidade) disturbs this equivalence. Understanding when the sight line still matches the drawn ray helps avoid systematic plotting errors in hilly terrain or windy conditions.

Given Data / Assumptions:

• Board is tilted about an axis perpendicular to the alidade.
• Alidade edge is used to plot rays directly on the sheet.
• Sights can be horizontal or inclined.

Concept / Approach:

When the board is perfectly horizontal, the line of sight defined by the alidade is parallel to the fiducial edge for any sight. If the board has cross-tilt, the line of sight will deviate relative to the plane of the board for inclined targets, breaking the intended parallelism. Only for truly horizontal sights does the projection remain parallel to the fiducial edge despite the cross-tilt, because the vertical component that would project differently is zero.

Step-by-Step Solution:

Verification / Alternative check:

A simple model using vectors shows that the component normal to the board introduced by tilt alters projected direction for inclined rays but becomes zero for horizontal rays, confirming the conclusion.

Why Other Options Are Wrong:

Inclined upward or downward: both create nonzero vertical components whose projections are altered by the cross-tilt.

None of these: incorrect because horizontal sights are an exception where the equivalence still holds.

Common Pitfalls:

Assuming rough levelling is sufficient for steep sights; forgetting to re-level the board after moving; using the alidade as if the board were perfectly horizontal in windy or uneven ground conditions.

Final Answer:

Horizontal sights