See what one app reveals about Manhattan power outages and cell service

The popular location-tracking app FourSquare, which "checks" users into certain locations based on GPS, shows the areas most impacted in Manhattan after Hurricane Sandy struck New York City four days ago.

This image gives a visual representation of how many people were "checking" into establishments and where before Sandy hit. A few days after the storm shows a stark difference with a hole right over Lower Manhattan, which was one of the areas most drastically impacted by flood waters and power outages.