# How to calculate the diameter of a moving circle from afar while lacking and knowing some information?

Let's say there is a solid wood wheel with two tires attached at the bottom travelling on a level road towards me while facing me width-wise (as in, the side of it from which you can see its diameter). I take a few camera shots of said wheel as it is moving.

I do not know its actual diameter, and do not know the distance between me and the wheel, but, I do know the speed it is travelling at, the field of view, and its angular diameter in each shot.

• Anyone else got an answer? – 1234567 Feb 20 '16 at 0:00

## 1 Answer

Knowing the speed, after known time you get a known distance traveled. Does your camera time stamp the photos? Compare that distance in your frame to the wheel diameter.

• No, it does not time stamp the photos. – 1234567 Feb 19 '16 at 23:06
• Then you need to either take a video, knowing the number of frames per second, or record the time between pictures. – Ross Millikan Feb 20 '16 at 3:10
• Is there really no other way around this? – 1234567 Feb 20 '16 at 14:11
• You are trying to get the scale of the photo. As you said you knew the speed I was trying to use it to set the scale. If you have an object of known size in the photo, like a person, you can use that to set the scale. Otherwise, I don't see a way to distinguish a small near object from a large far one. – Ross Millikan Feb 20 '16 at 14:15
• So, you're saying that, if, in each photo, I had a person of a known height who is close enough to the camera to have their body fully shown and nothing more, by somehow comparing his height to the circle's diameter as it approaches, I could calculate its real size? – 1234567 Feb 22 '16 at 21:16