VIDEO MEASUREMENT OF LINEAR DISPLACEMENT ALONG AN OBLIQUE LINE USING THE CROSS-RATIO

  • Mark Colpus
  • Mark Goss-Sampson
Keywords: cross-ratio, camera, calibration, perspective

Abstract

The 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.
Published
2016-05-05