VIDEO MEASUREMENT OF LINEAR DISPLACEMENT ALONG AN OBLIQUE LINE USING THE CROSS-RATIO
Keywords: cross-ratio, camera, calibration, perspective
AbstractThe purpose of this study was to rediscover the cross-ratio and assess its effectiveness for measuring linear displacement when the image plane is not parallel to the object plane. In the laboratory a fixed, 4m object length was reconstructed with a mean absolute error of 2.6mm (s.d. = 1.6mm, maximum = 4.9mm). In the field, two cameras filmed a fixed, 8m object length with a mean absolute error of 13mm (s.d. = 5mm, maximum = 20mm). The method is very accessible to non-specialists in projective geometry and the results are both valid and reliable.
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