What Is GPS?

Its full, unglamorous name is Global Positioning System. Launched by the U.S. military in 1990, GPS is a network of 24 global positioning satellites that orbit the planet, beaming radio signals back to Earth to receivers in cars, boats, planes, and hikers' hands.

How Does It Work?

After requiring radio signals from three or more satellites, a GPS receiver can triangulate your position and display it on-screen as a set of coordinates. Once it picks up four or more satellites, you can get your position in three dimensions, including altitude. It's the world's most precise way to navigate; the typical accuracy of a hiker's unit ranges from 3 to 30 meters.

What GPS Can Do

• Provide an exact geographic fix that can be plotted precisely on a map.
• Tell you the straight-line distance and direction to your destination.
• Record the day's travel as a "track," creating a highly accurate bread crumb trail you can reverse and follow home, or transfer to computer.
• Tell your altitude within 30 feet.
• Provide detailed trip information, such as mileage, speed, and elevation gain.
• Warn of topographical roadblocks like rivers and deep canyons, as long as you've loaded topo maps onto the unit.

What GPS Can't Do

• Provide enough map detail to plan long-distance routes or navigate through tricky terrain (pack topos or use our Adventure Planner mapping software).
• Warn of detours due to recently rerouted trails, fallen bridges, or natural disasters (call ahead or check recent trip reports on our site).
• Replace basic navigation skills (get a book or friend to teach you how to use a magnetic compass, read a topo map, and plot a route through terrain).

Jargon

Datum: Also called map datum. Every map has a "datum," which describes the survey grid used to match the coordinates and features on the ground. Most topos maps are in WGS 84 or NAD 27. Always match your GPS with your map's datum, located on the bottom of the map. Caution: If you're using a NAD 27 map and WGS 84 on your GPS unit, you could be off as much as 1/4 of a mile.

Latitude/Longitude: A traditional standard for representing your position on an east/west, north/south grid. Maps display this grid along their perimeter.

Point of Interest: see "Waypoint"

Route: A file of linked waypoints saved in your GPS unit or computer. Not as detailed as a track.

Track: Also called a tracklog. It's a series of tightly recorded waypoints automatically recorded by the GPS from a bread crumb trail of positions you've passed through since powering up. Displayed on your GPS screen, the track allows you to reverse your course of travel. It can be transferred to our Adventure Planner mapping program or other software on your computer for an exact plot on a map. Our map correspondents set their GPS units to collect a track point every 1/100th of a mile.

UTM: UTM is a reference grid that divides topographic maps into 1-kilometer squares for easy and accurate plotting. It's replacing latitude/longitude as the standard for modern land navigation.

Waypoint: Also called a Point of Interest (POI). It's an electronic pinpoint of a place.

WAAS: Wide Area Augmentation System is a network of ground stations that work with GPS satellites to enhance signal accuracy.

Tips For The Trail

• Although most units last 10 hours on two AAs, always carry spare batteries.
• To save juice, turn off the electronic compass and use the old-fashioned magnetic kind. Set the backlight time to its shortest setting.
• If you're not interested in data collecting or route plotting, only power up when marking a waypoint or double-checking your position. You won't get a bread crumb tracklog, but you may not need it.
• Save your waypoints in sets as distinctly named points for easier filing, storing, and uploading to your computer. It's easier to add notes to those waypoints as well.
• If you're using tracklog functions (letting your GPS store track points as you walk), make sure your GPS' off timer is disabled, so the unit won't automatically shut down.
• When using the "Go To" function, allow the directional pointer to reorient itself after you stop or detour. To avoid waiting, use your handheld compass to follow the correct bearing while the GPS resets.
• When navigating gets difficult, use all your tools at once: GPS, a handheld compass, and a UTM-gridded map.
• For serious orienteering, a 7.5-minute top map is indispensable. Downloadable topos for your GPS screen, even the best ones, lack fine topo detail and features like vegetation shading.

Thanks to Backpacker Magazine for "Using GPS" reference information.