There are small uncertainties in these measurements due to inherent limitations both of the photography and of the PPI display. The geometric scope centre can be determined from the bearing ring, but the range rings show a perceptible distortion from perfect circular symmetry and are not perfectly concentric (see below). Also the print focus appears to be of variable sharpness, and/or the video gain is varied. There is also variable noise speckling in the altitude hole. Thus the rings thus have some perceptible thickness and often appear faint and broken, sometimes vanishing, which introduces uncertainty. Original range and azimuth values used were based largely on careful measurements made by Brad sparks, using the nearest parts of what were believed to be 1-mile range rings to minimise distortion effects. But further research has required revision of the PPI range scale from 1-mile rings to 0.5 mile rings, and moreover because the display trace time is triggered not at zero range but at a range corresponding to the edge of the TR (transmit/receive) hole, given as at least 2000 ft in training manual CDC 32150K, Vol.4, ranges measured from the scope centre are therefore increased by a "hidden" radius. The effective zero-point of range is 0.25 NM, such that (for example) the third, brighter, 1/2 mile range marker is actually not at 1.5 NM but at 1.75NM, as shown in Fig.3. Values cited are believed accurate within error bars of +/- 0.05 mi and +/-1 deg.

The cause of the display eccentricity is probably optical. The range rings are electronically produced by brightening the scope trace and may be subject to instabilities in the video voltages or changes due to local electromagnetic or geomagnetic fields; but the direction of the eccentricity, diametrically opposite to the anomalous bright patch at around 26 degrees on every photo, suggests that the distortion is due to the angle of photography and convexity of the tube face. This would be consistent with Richard Haines' identification (unpublished private report) of the bright patch as an offset ghost image of the bright centre-spot, doubly reflected via the camera lens.

There is a complication, however, because the camera optics is understood to consist of a system of prisms and a lens mounted inside the CRT, the optical axis passing through the middle of the drive gear that rotates the tube inside its yolk to maintain "north-up" or "heading-up" presentations. (A second optical system combines an image of the clock, counter etc. onto the same film frame.) The image is therefore a reversed view of the inner, convex, phosphor-coated surface of the tube face, not of the outer, convex glass surface. Haines' theory therefore perhaps needs reinvestigation. Nevertheless the bright patch in question is almost certainly an artefact of an analogous kind.