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GPS photo tagging, also known as geotagging, is the process of embedding a digital photo with latitude, longitude and even altitude data.

­When people talk about "a GPS," they usually mean a GPS receiver. GPS is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails).

Each of these solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.

When you take a photo with a digital camera, the camera records a lot more data than just the image. This information includes the time and date when the photo was taken, the orientation of the camera (portrait or landscape), whether a flash was used and even detailed camera settings like aperture, exposure and focal length. All of these data is stored in something called the EXIF header of the photo.