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### 2. Reference Frame

The origin of the photo coordinate system is at the intersection of straight lines connecting opposite side fiducial marks. This intersection should coincide with the principal point, the location where the optical axis intersects the focal plane. However, the actual principal point, O_{C}, might not coincide with fiducial line intersections; it could be offset in the x and y directions and the axes rotated, Figure G-5.

Figure G-5 Principal Point Error |

Measured photo coordinates should be be adjusted for the offsets and axes skewness.

If calibrated coordinates of the fiducials are available, point coordinates can be determined by measuring from two (or more) fiducials, Figure G-6, then applying trigonometry to determine corrected coordinates.

Figure G-6 Coordinates from Fiducials |