Photogrammetry — definition of the principal line on an aerial photograph On an aerial negative/print, the principal line is defined as the straight line that joins the principal point to which of the following?

Introduction / Context:
Special points and lines on a photograph—principal point, plumb (nadir) point, isocentre, principal line, and horizon line—form the basis for tilt corrections, relative orientation, and metric measurements in photogrammetry. Misidentifying these features leads to systematic errors in plotting and restitution.

Given Data / Assumptions:

• We deal with a tilted (or potentially vertical) aerial photograph.
• Principal point: foot of the perpendicular from the camera’s perspective center to the image plane.
• Isocentre: midpoint of the line joining the principal point and plumb point.

Concept / Approach:
The principal line is the straight line on the photo joining the principal point and the isocentre. On a vertical photo, the principal point and plumb point coincide, making the isocentre coincide as well; in that special case the principal line is undefined in direction but any line through the principal point would coincide. On a tilted photo, the three points are distinct and the principal line has a definite direction used for orientation tasks.

Step-by-Step Solution:

Verification / Alternative check:
Geometry diagrams in standard photogrammetry references depict the principal line as PP–Iso, with the plumb point lying on the same straight line on the opposite side of the principal point when tilt exists.

Why Other Options Are Wrong:

• Nadir/plumb point: lies on the same line but the definition of principal line is PP to isocentre.
• Perspective centre: not a point on the image plane.
• Horizon vanishing point: different construction unrelated to the principal line definition.

Common Pitfalls:
Confusing the plumb point with the principal point in vertical photography; mixing up horizon line and principal line.

Final Answer:
Isocentre