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Using a GPS Unit
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The Global Positioning System:

About 20 years ago, the Department of Defense developed the Global Positioning System, a high-tech simulation of the ancient method of navigatiing according to the position of the stars. They launched 25 satellites into orbit. These satellite "constellations" have a steady orbit, so GPS receivers use them as fixed points from which to determine their own position. The satellites transmit coded radio signals which are picked up by GPS receivers. When a GPS receiver locks onto the nearest satellite, it determines how long it takes for the coded signal to reach it. Using this figure, the GPS calculates its physical distance from the satellite. With a distance reading from at least three satellites, a GPS unit can pinpoint its current position on the earth, similar to the way a hiker determines where he is on a map by referencing three or more distinct locations. This is called triangulation. With a reading from a fourth satellite, a GPS can determine altitude.
The government's primary purpose for GPS is as a navigationonal system for military use. It is also widely used for surveying and mapping by industries that rely on position information. With the new hand-held GPS receivers, GPS satellite information can be accessed by anyone - backpacker or sea kayaker, pilot or mountaineer. With this device, anyone can find a location in a matter of minutes and plot a route with several waypoints.
To some, a GPS unit may seem like just another gadget to add to the closet full of outdoor gear. True, it may not be an essential item for most backpacking trips, but for the serious mountaineer, curious backpacker, or orienteering enthusiast, it can be a good supplement to the maps and compass. By getting a location fix on your favorite spots, the coordinates can be marked on your map for a quick reference for a return trip.
Plotting a Route with GPS:
Typical GPS Unit

When using a GPS for the first time always refer to the GPS instruction information. Initialization may need to be repeated if the GPS is moved more than 300 miles while it is turned off.

To plot your route ahead of time, use a map to determine a set of appropriate coordinates (called waypoints) along the planned route/trip. Give each waypoint a name that is recognizable or unique. A great planning tool is National Geographic's TOPO! software. TOPO! provides seamless topographic coverage of entire states, and enables you to trace out a route, create an elevation profile, choose and name waypoints, and upload trail information directly into the GPS.

Latitude and longitude as well as UTM coordinates are located along the map border of National Geographic Trails Illustrated maps. Selected GPS waypoints are also shown. When used with map datum NAD27 (North American Datum 1927), these coordinates represent an approximate location fix. National Geographic Maps show the coordinates of prominent places, such as trailheads, roads, lakes, emergency locations, or water sources, for your convenience. If a National Geographic Maps waypoint is input into a GPS, a bearing and a distance toward that position will be given.
Waypoint for Kokomo Pass
Traveling along the planned route, each time the GPS is turned on it will give coordinates for the current position. A GPS will also provide a bearing and straight line distance to the next waypoint. This information is more useful when used in conjunction with a map.
Waypoint coordinates
When the GPS provides a bearing, it allows for easy orientation to the next waypoint along the route. Since the GPS distance reading is a straight-line distance, which is often not the most convenient route to travel in the backcountry, it is important to be able to interpret the topography of the land that lies ahead before the next waypoint. For example, a lake may be 3.5 miles away as the crow flies, but the steep, exposed switchbacks and the 1500' elevation gain on route to the lake must be considered.
The fact that a GPS can pinpoint a near exact location is really the most valuable feature for hikers and other land-based, recreational users. When the weather is stormy and visibility is limited, a GPS can provide coordinates of a fixed position. Once these coordinates are located on a map, and a quick evaluation of the
GPS Data on TOPO! Map
topography and surrounding area is made, it can then be decided whether it is better to move on, turn around, or remain in the current location. In some cases, such as less than ideal satellite position, a hand-held GPS position fix can be off by 15 meters or more. 15 meters is less than 0.02 inches or 0.5 mm on a Trails Illustrated map at a scale of 1:32,400. This margin of error is really quite insignificant in most cases.
A GPS needs to have a "clear view" of the sky in order to give its most accurate reading. Thick forest and narrow canyons may obstruct the receiver's view of the satellites, and it does not function well indoors. Cold weather may also affect the LCD screen and battery life.
Other Features:
Most GPS units can display your speed of travel and the estimated time of arrival at your destination. Some GPS units will even display the position of the sun and the moon. A GPS can monitor a traveled course over ground, or the direction traveled, and it will identify travel errors, such as crossing back over the traveled route. GPS units rely on batteries, and to avoid batteries dying during a trip, it's a good idea to turn the GPS only when it is needed to check navigation. Be sure to carry additional batteries on long trips.
This article is based on the "Using a GPS Unit" chapter of the National Geographic Basic Map & GPS Skills booklet, reprinted here by permission of National Geographic Maps.