Template:Cross ratio metrology example.svg

Use of cross-ratios in 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.

1.	The width of the side street, W is computed from the known widths of the adjacent shops.
2.	As a vanishing point, V is visible, the width of only one shop is needed.

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