Summer is for family vacations, share houses at the beach and theme park outings with friends. But how to keep track of a big group of travelers without a flurry of texts and calls? Are the children at the pool or the arcade? Who is already on the beach? Is everyone en route to the Ferris wheel or the Tilt-a-Whirl?

Today it's easier than ever to find out, thanks to the proliferation of GPS-enabled smartphones and a steady stream of free or affordably priced apps. The number of active users of "family locator services" (smartphones with GPS and personal location devices) in North America and Europe is expected to reach 70 million in 2016, up from 16 million in 2011, according to a new report from Berg Insight, a research firm based in Sweden.

Certainly, you can keep tabs on fellow travelers using other methods, including cellphone programs (like AT&T's FamilyMap and Verizon's Family Locator) that let you keep a distant eye on family members, and tracking devices like the battery-powered GPS Atlas F100 and the Toddler Tag Child Locator, which uses radio frequency technology (your child gets a tag; you get a key chain transmitter). The good old walkie-talkie deserves an honorable mention (useful in places with spotty cell service like the backcountry or cruise ships). And, while it's not a tracking solution, I'll give a nod to Safety Tat customizable temporary tattoos: child-friendly graphics (dinosaurs, butterflies) alongside the phone number of your choice and the words "If lost, please call." (Also a great gift for adults with existential anxiety.)

That said, tracking apps give you the most bang for your buck. You can locate your brood on a map without ever making a call, receive alerts if the kids venture beyond a designated area, and turn your smartphone into a newfangled walkie-talkie. Below, a guide to the apps that can keep you together, even when you're apart.