Caution: GPS devices aren't always right

— -- The distress call came in to Sgt. Christopher Buckley late one night: Two motorists and their families had followed their Global Positioning System devices off the main road near Windham, Vt., onto a snowmobile trail. Though the road became narrower and the snow deeper, the two men kept driving.

"Then they ran out of road, and they sunk in the snow," Buckley, of the Vermont State Police, says of the February incident.

It's not the first time drivers have blindly followed their GPS units, Buckley says. He's assisted on three rescues since January, and others in his division have logged at least three other instances in which people drove over snowbanks onto unplowed roads and got stuck.

One Fort Drum, N.Y., soldier drove his car over a snowbank and sank so deeply onto a trail that police needed a snow tractor to haul him out, Buckley says.

"I ask them why and they say, 'That's what my GPS told me to do,' " says Buckley, who late last month issued a news release pleading with drivers to trust their common sense over their navigation devices.

As GPS units and their databases of maps become common features in cars and cellphones, more drivers are encountering unmapped areas and map inaccuracies, says Andrea Barisani, chief security engineer for Inverse Path, a computer software company based in England.

In response, some companies are trying to map these obscure nooks, unpaved roads and trails. Others are asking customers to correct inaccuracies and report changes on their routes.

"All of those maps have very big gaps in them, and there's a whole cottage industry developing that's trying to fill in those gaps," says Marcus Needham, president of Mountain Dynamics of Tucson, which launched a GPS device last December with a database that includes 100,000 miles of snowmobile trail maps. The company also mapped 75 North American and 420 European ski resorts.

TomTom, which manufactures a GPS device, has asked its users to make corrections directly on its units' touch screens, such as editing a street name when they spot a mistake, says Tom Murray, a spokesman for the Concord, Mass., company. TomTom vets the information and updates its database.