GPS receivers add twist to hide and seek

HAVRE DE GRACE, Md. — If you hide it, Lynn Black will find it.It doesn’t matter whether it’s an old ammunition box hidden in a cemetery or a rock made famous by the movie “The Blair Witch Project.”

Mrs. Black, of Middletown, Pa., has logged more than 7,000 finds with her Global Positioning System receiver since she became hooked on the sport of geocaching more than three years ago.

“She was the type that you could never take her outdoors before, and now I can never keep her at home,” says her husband and sometime geocaching partner, Kevin Black.

And here at Susquehanna State Park, where the Maryland Geocaching Society has assembled 50 to 75 avid cache-hunters for its annual fall picnic, Mrs. Black is on the hunt again.

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Geocaching is a high-tech scavenger hunt that uses the Global Positioning System and the Internet to direct geocachers to a stash. Players “cache,” or hide, items in various locations and publish coordinates so other GPS users can find them.

Each cache is described in a page on the sport’s virtual home base at www.geocaching.com. The page contains the coordinates of the cache location (in longitude and latitude), an area map and a brief description of what to look for. Some also contain encrypted clues and a key to decode them. The clues are encrypted because some geocachers prefer not to use them.

The game is played worldwide, from Afghanistan to Antarctica, from Vatican City to Malawi, and often inspires social events. Visitors to www.geocaching.com can see, for example, that geocachers in Sligo, Ireland, met last weekend for “a friendly morning cup of coffee” and a hike to two caches hidden at the top of Ben Bulben, the mountain made famous in the poetry of William Butler Yeats.

The fast-growing sport dates from May 1, 2000, when restrictions on the Global Positioning System — originally developed for military navigation — were removed, allowing the manufacture and sale of commercial receivers accurate to within 20 feet.

The GPS receiver, a device about the size of a cell phone, receives navigational signals from satellites orbiting the Earth. Its display shows distance and direction to the destination programmed into the unit.

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The basic rules of the sport are simple: Players hide containers known as “caches” at various locations.

Each cache contains a logbook and perhaps some kind of prize or item designed to be passed from player to player. Those who find the cache are supposed to sign the logbook, take something from it and leave something else in return.

“It sort of becomes a swap meet out in the woods,” says Jeremy Irish, a partner in Groundspeak, the Seattle-based company that runs the sport through www.geocaching.com.

Mr. Irish, a Web developer and programmer, was attracted to geocaching by its flexibility, he says by phone from Seattle. For the adventurous, the game can be combined with other outdoor activities, such as kayaking, rock climbing and mountain biking, or it can be simple enough for children to play in an urban park.

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