A method of visual metrology from uncalibrated cameras is proposed in this paper, whereby a camera, which captures two images separated by a (near) pure translation, becomes a height measurement device. A novel projective construction allows accurate affine height measurements to be made relative to a reference plane, given that the reference plane planar homography between the two views can be accurately recovered. To this end a planar homography estimation method is presented, which is highly accurate and robust and based on a novel reciprocal-polar (RP) image rectification. The absolute height of any pixel or feature above the reference plane can be obtained from this affine height once the camera?s distance to the reference plane, or the height of a second measurement in the image is specified. Results from our data show a mean absolute error of 6.9mm and with two outliers removed this falls to 1.5mm.