# How to accurately calculate size in image using relations and a reference

OK so the relation math is easy enough.

If you know the size of an object, say a 12" ruler in the image, and you put the ruler flat against another object like a door. You take a picture of the door with the ruler.

The ruler is 12in, and is found to be 120px tall in the image. the door is found to be 1200px, which when you do the easy math, is 10ft, or 120in.

However that is not how it works, in reality the image has some form of distortion, so a direct relation will not work. It has something to do with gathering light, and focal points/lengths or something. What do I need to account for to make this relationship work?

what we have so far

(12in / 120px) = (unknownHeight / 1200px)
(12in / 120px) * 1200px = unkownHeight
0.1ppi * 1200px = 120in or 10ft


But in reality if the ruler is 12in and 120px, the 10ft door will be like 1100px, so we need to account for like the lens or something?

What am I missing?

Thanks!

• google "photogrammetry" and begin your journey into the complex world of "the science of making measurements from photographs." Apr 25, 2014 at 19:20