Calculating Distance using Video Analysis

There are many biomechanical measurements that can be made using video analysis. Described here is a simple method for measuring distance that nearly anyone with a basic camera and TV can do. There are also several Video Analysis products that can do this and much more.

Make note of the video technique guidelines before collecting video footage for analysis. Note that for distance measurement on the video you will need to have in the view of the video a standard distance measure for calibration, placed in the same plane as the object being measured. This can be a measurement stick (e.g. yardstick) that you hold up in the video frame, or markings on the ground a known distance apart.

Calibrate for distance by determining the length of the calibration measure on the screen. It is just like using a scale measure on a map. If a three foot (36 inches) distance is one inch on the screen, the calibration ratio is 1:36. Everything in the same plane as the calibration measure will be 36 times bigger than what you can measure on the screen. If the object is either closer or further away from the camera than the calibration measure, then the scale will be different and the distance calculated inaccurate.

Now simply measure how far an object moves on the screen, and multiply that by the calibration ratio. An easy way to do this is to place a clear plastic sheet over the screen, and skip through each frame one by one, marking on the plastic sheet key points that can be measured later.

If the frame rate of the video is known, you can then calculate speed.

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