View source for Template:Cross ratio metrology example.svg

[[File:cross_ratio_metrology_example.svg|thumb|{{{1|400px}}}|Use of [[cross-ratio]]s in [[Projective geometry|projective geometry]] to measure real-world dimensions of features depicted in a [[perspective projection]]. A, B, C, D and V are points on the image, their separation given in pixels; A', B', C' and D' are in the real world, their separation in metres.<ul>
<li>In (1), the width of the side street, W is computed from the known widths of the adjacent shops.</li>
<li>In (2), the width of only one shop is needed because a [[Vanishing point|vanishing point]], V is visible.</li>
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