GPS: What you need to know

FRESNO, Calif. In October of 2009, an emotional reunion came three days after three hikers became trapped on a ledge in Kings Canyon National Park. A search and rescue helicopter crew spotted a fire the men set and safely brought them down to their anxiety filled family members.

Father of rescued hikers, Wayne Zeman said, "Seeing that helicopter land, knowing over the radio it was them gave a description of who they were, where they were, it was emotional."

Rescues like this one happen hundreds of times a year in the mountains of Central California, where the terrain can be treacherous.

In Yosemite National Park, search and rescue rangers have saved many people from steep cliffs, raging rivers, and deep snow. They've seen it all, and now they're seeing more cases of people putting themselves in precarious positions because they relied too heavily on one type of technolog ... GPS.

Valley District Ranger, Eric Gabriel said, "More and more common, and what happens is that people rely solely on the GPS as opposed to having a map, having a compass, and good judgment and skill to use those things."

Gabriel says in one recent case a hiker using only a global positioning system wandered off a trail and was lost for eight days. Fortunately he had packed plenty of food and water.

"They were prepared for long distance backpacking. And so that had a happy outcome," said Gabriel.

Many hikers who use GPS say it can be a very useful tool in certain situations.

Yosemite visitor, Sonny Lawrence said, "On big mountains, just as you can see here today, a white out can come in, in which case map and compass doesn't do any good."

Yosemite visitor, Joshua Hensley said, "It's useful if you're not sure what trail you're on, you can make sure you stay on the correct trail."

But these devices do have limitations ...

The John Muir Trail, it's the busiest trail in the park with as many as 2-3000 visitors a day in the summer. But it's also a place where hikers can run into trouble with their GPS because of the thick tree cover overhead."

Yosemite Ranger, Paul Doherty said, "The general rule is if you can't see clear sky, your GPS can't reach the satellites."

There's also the issue of running out of battery power. And GPS can't tell you when a stream has washed out a trail or when severe weather is rolling in.

"It can't get you out of danger. It can't call for help. And it can't tell you anything more than where you are," said Doherty.

That's why rangers say it's important to take other steps before heading out on a hike.

Gabriel said, "Let people know where you are, what your intended plan is and when you're going to be back. It's extremely important to have the food, water and equipment to be out and be safe."

Gabriel says proper preparation is key to taking in the sights without taking an unnecessary risk. "We want people to come up and enjoy the grandeurs of this beautiful place and get home safe."

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