APPENDIX FIVE

HIKING WITH THE GLOBAL POSITIONING SYSTEM (GPS)

To gather trail information and navigate, a basic GPS unit such as the Garmin Etrex or Magellan Sportrak is a relatively simple and effective tool. Even if you have not used a GPS unit or have limited experience, the basics are easy to master. Prices for a handheld unit range from $100 to $500. Places to purchase units include the Internet, electronics stores, and even large chains such as Wal-Mart.

The primary objectives of a GPS unit are to collect and track data as you walk and to be used as a navigation tool to reach a specific location. The data you collect while hiking can be downloaded later onto a software program such as TopoUSA ($99 for topos of the entire United States) and will overlay the path you walked on top of digitized topo maps. The trail is displayed as you hiked it and may be edited online.

A Global Positioning System (GPS) unit receives data from 24 satellites that orbit the earth. Originally for military use only, the technology is now available for civilian use. When you turn on your GPS unit, it searches the sky and locks onto as many satellites as possible. The unit displays which satellites are available, the strength of the signal, and the location in the sky.

The unit must lock onto at least three satellites in order to accurately fix a moving 2-D position. With three satellites, latitude and longitude are available but not elevation. After locking onto four satellites, the unit is capable of triangulating your east/west, north/south position and altitude. Without WAAS, explained below, the unit is accurate to within 50 feet horizontally and 62 feet vertically. If your GPS unit is WAAS enabled, accuracy increases to 10 feet horizontally and 20 feet vertically.

WAAS stands for Wide Area Augmentation System. There are two WAAS satellites that receive corrective data from ground stations. This corrected data is then broadcast on the same frequency as that of the other GPS satellites.

Using GPS to Gather Trail Data

For most trails, you should be able to maintain a satellite signal lock for the entire hike. Occasionally, though, you will lose the signal. If you look down at the GPS screen and notice a gap in the track (trail) line, the unit has lost the signal but picked it up again. This can be corrected by joining the lines to fill the gap, once the track data is downloaded. Areas you will have problems locking onto signals include hikes next to sheer walls, hikes that meander between tall buildings, and hikes that pass beneath dense tree cover.

In the case of a hike where it is impossible to keep a locked signal, you should turn off the tracking feature and simply plot waypoints whenever the signal appears. Once downloaded onto the topo software, you can draw the trail by connecting waypoints.

Using GPS to Navigate

A GPS unit is capable of leading you to a specific location. Prior to a hike you may want to upload waypoint or map data to your unit. With the target location in view on your screen, the simplest approach is to walk toward the destination. The cursor arrow, which represents the person holding the unit, will travel toward the target as you walk. If you wander off course, it becomes visually obvious as the track begins to veer away from your desired destination

You may also use the unit’s GoTo function. Select a waypoint or location from an uploaded map and direct the unit to lead you there. A steering screen guides the user toward the desired target. It will also indicate your bearing, how far you are off course, your speed, and will estimate your time of arrival.

For more advanced users, pinpoint data from a topo map can be entered manually into a GPS unit. The unit will then guide you to that point. This feature requires that you learn how to decipher easting and northing information supplied on topo maps. A good resource for this activity is GPS Made Easy by Lawrence Letham.