Prometheus 6

I can think os so many ways to frustrate the very attempt...like walking with someone close enough that your shadows overlap. Or on a crowded street. Or on irregular ground.

Shadow analysis could spot terrorists by their walk
04 September 2008
From New Scientist Print Edition.

Nearly seven years after Osama Bin Laden disappeared, US intelligence agencies are still chasing his shadow. And shadows are precisely what they should be looking for, says NASA's Jet Propulsion Laboratory in Pasadena, California.

By analysing the movements of human shadows in aerial and satellite footage, JPL engineer Adrian Stoica says it should be possible to identify people from the way they walk - a technique called gait analysis, whose power lies in the fact that a person's walking style is very hard to disguise.