Did you ever think about how great it is, if your new digital camera got GPS? In Apple’s Photos or any other modern image management software you can automatically see on a map, where the picture was shot.
Are you a big fan of Facebook, Twitter and so? You upload Pictures from the last party, friends, events, …?
You know, you have to protect you privacy.
Therefore you do not provide an address and a telephone number on online media. Well, that’s what you believe. You upload the picture. But the picture got meta information in the so called Exchangeable Image File Format (EXIF). In Exif you can get the information which camera, what aperture, which shutter time, what date and what time the picture was made. If the camera supports GPS, you get also the exact coordinates where the picuture was shot. To get the meta information of a picture use ⇒Exif-Viewer on a Mac,⇒exiftool on Linux and ⇒Kuso EXIF Viewer under Windows.
You can probably imagine, that these information is automatically evaluated. It is easily possible to extract the meta information from the pictures and to look for matching results.