  This paper addresses the high precision measurement of the distortion of a
digital camera from photographs. Traditionally, this distortion is measured
from photographs of a flat pattern which contains aligned elements.
Nevertheless, it is nearly impossible to fabricate a very flat pattern and to
validate its flatness. This fact limits the attainable measurable precisions.
In contrast, it is much easier to obtain physically very precise straight lines
by tightly stretching good quality strings on a frame. Taking literally
"plumb-line methods", we built a "calibration harp" instead of the classic flat
patterns to obtain a high precision measurement tool, demonstrably reaching
2/100 pixel precisions. The harp is complemented with the algorithms computing
automatically from harp photographs two different and complementary lens
distortion measurements. The precision of the method is evaluated on images
corrected by state-of-the-art distortion correction algorithms, and by popular
software. Three applications are shown: first an objective and reliable
measurement of the result of any distortion correction. Second, the harp
permits to control state-of-the art global camera calibration algorithms: It
permits to select the right distortion model, thus avoiding internal
compensation errors inherent to these methods. Third, the method replaces
manual procedures in other distortion correction methods, makes them fully
automatic, and increases their reliability and precision.
