Stereoscopic plotting: when the z-column of the right projector is raised, at which instrument positions does the model exhibit maximum y-parallax?

Introduction / Context:
Analog stereo-plotters generate a three-dimensional model from overlapping aerial photographs. Adjustments of projector columns (x, y, z) control relative orientation and parallax. Understanding where y-parallax maximizes helps the operator diagnose orientation errors and perform accurate model set-ups.

Given Data / Assumptions:

• A standard two-projector stereo-plotter is used.
• The right projector’s z-column is raised slightly from the relative-orientation condition.
• Instrument “positions” refer to standard checking locations around the model where parallax is assessed.

Concept / Approach:
Raising the z-column alters the image scale in the right projector, creating a differential vertical magnification that manifests primarily as y-parallax (parallax perpendicular to the air base). This effect is not uniform across the model and is known to peak at symmetric positions along the principal line away from the nadir, commonly noted as positions 4 and 6 in classical test patterns.

Step-by-Step Solution:
Start from correct relative orientation → y-parallax ideally zero at control points.Introduce a small +Δz on right projector → scale difference between left and right images.Scale difference translates predominantly to y-parallax that varies with location on the model.Empirically and from geometry, maxima are observed at the designated check points “4” and “6”.

Verification / Alternative check:
Operators verify by measuring y-parallax across a six-point pattern; largest values appear at 4 and 6 for a pure z-column perturbation.

Why Other Options Are Wrong:
Positions 1 or 2 alone do not capture the peak y-parallax created by a z-column change; single-position options (4 or 6 only) are incomplete.

Common Pitfalls:
Confusing x-parallax (related to base and height) with y-parallax (primarily tilt/scale differential); mixing up projector x/y/z adjustments.

Final Answer:
Position 4 and 6