: 2D dimensions from photograph Suppose I want to take the dimensions of an object of negligible width on a flat surface from a photograph and am able to calibrate in the sense that I know

@Pierce403

(Assumes Photoshop)

Measure your reference object.

(Use physical measurements—inch, mm, etc.—not pixels. The ruler tool is hidden under the eyedropper. You can also use the line tool, which is actually a little better in that it reads out next to your mouse, not on some distant info palette.)

Divide the real-world size of the reference object by the image size of the reference object. Multiply by 100 to get a percentage.

Go to Image->Image Size and scale the image by that amount. (You want resample off. The only catch is that Photoshop won't let you take the ppi below 1.0, so you might have to uprez before scaling.)

Go back and measure stuff. Things are actual size. (At least things co-planar with the reference object.)

If you're dealing with really huge physical objects (say, a measuring buildings on a satellite photo.) convert to something that's off by a power of ten to make the manual conversion easier.

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