A GPS Primer

By Neil F. Woodworth

For years, I considered global positioning system (GPS) receivers unreliable electronic gadgets unsuited to the rigors of backcountry travel. Today, though, I seldom hike, ski or paddle without my GPS receiver.

I was introduced to GPS technology when ADK began its Wild, Scenic and Recreational Rivers Act Survey project in 2005. I watched the leaders compile an electronic record of our travels and observations that could be downloaded to a computer and depicted on a digital topographic map. I was fascinated and decided to research my first GPS receiver.

I read GPS for Dummies, GPS Made Easy and Garmin’s online guide GPS Guide for Beginners. I lurked in the “Groundspeak” Web forum of the high priests of GPS, the geocachers. I learned that a personal computer (PC), a GPS receiver, and digital mapping software, used knowledgeably with paper topographic maps and compass, could make bushwhacking and backcountry travel more precise and more fun. A GPS unit never replaces a map and compass, but it does enhance wilderness navigation.

Time Out for a Glossary

  • A GPS receiver uses FM radio signals from orbiting satellites to determine a map coordinate position or “location fix” on the earth’s surface within twenty-five to fifty feet.
  • Location points are described in numerical degrees, minutes and seconds of an X/Y axis of longitude and latitude coordinates.
  • A single location point is known as a waypoint.
  • A series of waypoints plotted on a map to a destination is a route.
  • A track is a series of black dots on your GPS receiver’s map screen depicting your actual route that are recorded as you hike to your destination. You can “backtrack” to your starting point using these electronic breadcrumbs.
  • Heading is your direction of travel as you hike in compass direction terms, displayed in degrees or Cardinal letters (N, NE, E).
  • Course is your desired heading from starting point to destination.
  • Bearing is the compass direction from your ever-changing current position as you travel to your destination.

Points to Consider

I narrowed my choice to the most commonly available brands of handheld GPS receivers, Garmin and Magellan. Initially, I had to choose between a non-mapping GPS unit and a GPS mapping receiver. The former use a plotting screen to show your position, waypoints and intended route to your destination, while the mapping units depict the same information superimposed on a detailed topographic map uploaded from your computer at home. On the latter units, the map scrolls as you hike or paddle toward your destination, zooming in and out to show detail and area.

After your hike or paddle, you can download your hike’s track and waypoints from your receiver to display your actual travels on digital maps displayed on your computer. I strongly recommend Rich Owings’ excellent book on the use of digital mapping software with GPS receivers, GPS Mapping (Ten Mile Press, 2005).

Regardless of the mapping software that you use with your GPS receiver, make sure that the datum (map coordinate reference system) used on the map matches the datum selection of your GPS unit. Most paper topographic maps use a NAD 27 datum, whereas the default for most GPS receivers and digital mapping software is the NGS 84 datum. A datum mismatch can result in a position error of several football fields’ length or more in the woods.

Buyer’s Guide

I selected a Garmin GPSMap unit over a Magellan Explorist receiver with comparable features because the Garmin MapSource National Parks – East mapping software includes detailed 7½ minute (1:24,000 scale) topographic maps of the Adirondack Park.

I chose a color screen to take full advantage of the detail of these digital topographic maps.

My GPS unit connects to my computer via a USB cable which makes uploading maps, routes and waypoints quick and easy. I always carry extra batteries; I usually get 10 to 15 hours of use from a set.

Read All Instructions

In any bushwhack, the GPS unit will recalculate your position and your new bearing, heading, and distance from your destination.

The GPS receiver’s navigation display indicates your speed, course, heading, bearing, distance to the destination, your altitude and the estimated time of arrival at your current rate of advance. The map display typically includes your compass course and heading to your destination.

When you upload the topographic display maps, you also transfer a great deal of geographic information to your GPS receiver. When you press the FIND button, you get an option to search for previously loaded waypoints, routes, or nearby geographic features such as summits, lakes, ponds, or trails that are known in GPS lingo as points of interest (POIs). You can select to navigate from waypoint to waypoint or directly to a lake or mountain summit using POIs.

I carry the GPS receiver in a home-made holster secured to the shoulder strap of my pack with Velcro straps, leaving my hands free while giving the GPS receiver’s antenna clear access to the sky. I can reach it easily for position and bearing checks. But I’m still a Luddite at heart; my compass is around my neck and the paper map is in my pocket. E-mail me with questions or comments on this article.

GPS and Me: A Postscript

Several people who read my “GPS and Me” article (May/June) e-mailed me asking me about the performance of today’s GPS receivers under heavy tree cover or dense foliage. I have used my Garmin GPSMap 76CS unit summer and winter in a variety of Adirondack and Catskill tree canopy conditions and have never lost a signal.
Moreover, the latest Garmin “X” unit receivers, the GPS Map 60Cx, 60CSx, 76Cx and 76CSx models ( the added “S” means the unit includes an electronic compass and altimeter) incorporate the new SIRFSTAR III receiver chip with excellent GPS satellite signal reception under heavy foliage and even indoors. Based on my own experience, the Garmin “X” units put to rest worries about poor reception or signal loss under dense tree canopies.

Owners of Magellan Explorist units with 14 channel True-Fix technology claim similar performance. I can’t comment since I have had no experience with these Magellan units.

In my article, I discounted the value of an electronic compass integrated with the GPS receiver. After more field experience, I feel differently today. Your GPS receiver cannot tell you which direction you are facing (your heading) when you are standing still. The solution is configuring the GPS unit’s electronic compass to “power on” after you are stopped for a certain brief time interval, perhaps 10 or 20 seconds. Then, the electronic compass will display your heading with the black arrow on the map display and numeric display on the compass page in relation to your bearing to your destination. As soon as you begin walking again, the GPS function replaces the electronic compass. Used in this fashion, I have found the additional battery drain attributable to use of the electronic compass to be negligible.
--Neil Woodworth

Neil Woodworth is executive director and counsel for the Adirondack Mountain Club.