For those with dialed orienteering skills, learning to use a Global Positioning System, or GPS, can improve accuracy and efficiency. Coupled with digital mapping software, GPS can significantly ease trip planning and documentation. The result is a safer and more enjoyable backcountry experience that is easily shared with others.
If you lack basic map and compass skills this post is not for you. GPS is not a replacement for solid orienteering skills. It builds on them. GPS navigation is advanced navigation. A solid understanding of traditional map and compass navigation is needed to effectively use a GPS in the backcountry. The Mountaineering Club of Scotland offers excellent online instruction for traditional map and compass orienteering skills.
Global Positioning System
Global Positioning System (GPS) was developed by the United States’ Department of Defense. The current GPS satellite constellation includes 24 to 32 Medium Earth Orbit satellites maintained by the United States Air Force. Each GPS satellite continually transmits precise microwave signals that include the satellite’s exact location and the precise time it was at that location. By subtracting a satellite signal’s time from the current time, a GPS receiver can compute its distance from each satellite.
The process a GPS receiver uses to determine its latitude, longitude and elevation from the location and distance of satellites is called a trilateration. While in theory three satellites are enough to calculate a position, even a miniscule clock error multiplied by the speed of light (the speed at which satellite signals propagate) results in a large positional error. Therefore, a GPS receiver requires at least four satellites to accurately determine its location.
Direction and speed can then be calculated from known changes in location. A GPS receiver can record this information and present it in numerical units or within a moving map display.
A coordinate system enables every location on Earth to be specified by a set of numbers. GPS receivers are capable of using many types of coordinate systems and it is essential to understand what coordinate system is best and how to use it.
For land navigation, coordinate systems primarily involve a horizontal position that includes latitude (north or south) and longitude (east or west). A vertical position or altitude (elevation or depth) may also be included. Altitude is of greater importance in aviation.
Latitude specifies the north-south position of a point using lines that circle the earth parallel to the equator. Utilizing imaginary lines running between Earth’s two poles, longitude specifies the east-west position of a point. Altitude indicates a vertical location with sea level being the most common reference point.
Both latitude and longitude are commonly expressed numerically as degrees. Measured from the center of the Earth, these degrees are based on the angle between the Equator (latitude) or Prime Meridian (longitude) and a point on Earth’s surface. Using this measurement, for example, the north and south poles form right angles with the equator (0°) and are therefore expressed as 90°N or 90° S, respectively.
The Prime Meridian (0°) and the 180° Meridian form a “Great Circle” and divide Earth into the Eastern and Western Hemispheres. Rooted in international convention, the Prime Meridian passes through the Royal Observatory, Greenwich in southeast London. The 180° Meridian is the bases for the International Date Line. Degrees longitude is expressed in values ranging from 0° to 180° East or West, relative to the Prime Meridian.
Like units of time, degrees latitude and longitude are typically divided into 60 parts or minutes. Minutes are further divided into 60 parts or seconds. Alternatively, degrees and minutes are sometimes divided into tenths, and expressed as decimal degrees or decimal minutes.
Universal Transverse Mercator
The metric system equivalent for navigation is known as the Universal Transverse Mercator or UTM. This is a popular coordinate system that is relatively easy to understand and communicate. Most topographic maps, including USGS maps, show UTM coordinates and often a UTM grid. The system uses meters as its basic measurement unit, making distances easy to calculate.
In the Northern Hemisphere latitude is expressed in meters north of the equator. To avoid negative numbers in the Southern Hemisphere, the equator is set at 10,000,000 meters and the value for latitude - called the northing - decreases southward. There is no south.
Longitude is broken into a series of 60 overlapping zones (each 6° degrees wide) with their own central meridians. Longitude is expressed in meters east relative to each zone’s central meridian and is called the “easting”. To avoid dealing with negative numbers, the central meridian of each zone is set at 500,000 meters east. There is no west.
A location using the UTM system includes a zone number and the “easting” and “northing.” Longitude or the easting is always expressed as a six-digit number and latitude of the northing as a seven-digit number.
A geodetic datum is a set of reference points on Earth's surface against which position measurements are made. It defines a particular geographic coordinate system. It is important to set the datum of a GPS receiver to match the map being used. The location error, if they are not matched, has the potential to be significant. It is standard for professionally generated maps to list the datum. In North America the most common datums include NAD27, NAD83, and WGS84. Maps from international sources may use many alternative datums.
The North American Datum of 1927 (NAD27) is a referencing system for North America based on a survey station at Meades Ranch, Kansas. Prior to satellite technology, NAD27 was commonly used and is still the datum for older USGS topographic maps but is superseded by the North American Datum of 1983 (NAD83). NAD83 was developed after advances in technology provided more accurate information on the actual shape of Earth.
The World Geodetic System of 1984 (WGS84) is the datum used by the U.S. Department of Defense for mapping, charting, surveying, and navigation. The WGS84 datum, which is almost identical to NAD83, is the only world referencing system in place today. WGS84 is the default standard datum for all GPS receivers.
Navigating the AMWSC with a Garmin Foretrex
Photo, Luc Mehl
Options overwhelm the handheld GPS receiver market. And if you like spending money it is an easy place to do it. Most handheld GPS receivers are over designed with many more functions and options than you need. Gee whiz selling features too often obscure the basics of what really matters. Save yourself some money and hassle and choose a GPS receiver that is simple, durable and easy to use. My advice - Keep It Simple Stupid!
Currently my favorite GPS receiver is the Garmin Foretrex 301. This slim, light, and waterproof unit uses standard AAA batteries. The relatively inexpensive Foretrex comes with a bulky Velcro wristband that I replaced with simple strap that I wear around my wrist or attach to a shoulder strap of my pack. USB interface cables are available for the Foretrex.
GPS receivers with larger full-color display screens can be helpful — I appreciate them when navigating in a car. For backcountry navigation, however, printed maps remain necessary, and large GPS display screens are redundant. That said, I confess to being intrigued by the Garmin GPSMAP series of receivers. While on the heavy side (around 9 ounces), they sport a waterproof design, use standard AA batteries, have respectable battery life (20 hours), and come with a USB interface cable.
When on adventure mountain bike rides, where referencing a printed map at every route-finding decision is highly inconvenient, I do appreciate a GPS receiver that displays some basic map information. The reasonably priced Garmin eTrex Legend is one such model, and can be mounted on my handle bars. While the eTrex Series is not waterproof, they are fairly rugged. The eTrex Legend is light, easy to operate, uses standard AA batteries, and has the option of a USB compatible interface cable (sold separately).
The Garmin Forerunner series and the Suunto Ambit and X10the newest generation of GPS watches by manufacturers with a record of producing quality products. The Forerunner and Ambit are designed primarily as athletic training tools that track pace and heart rates as well as time and distance. The expensive X10 is designed primarily for navigation. The X10 comes with a computer interface cable and is compatible with National Geographic Topo!. The Forerunner, Ambit, and X10 all run on rechargeable lithium-ion batteries that cannot be replaced in the field.
Providing an absolute location or position is the most basic function of a GPS receiver. The location, displayed as coordinates, maybe manually plotted on a map or recorded for later reference. In an emergency rescue or evacuation the location can be relayed to others.
A waypoint is a set of coordinates (including longitude and latitude) that identify a point in physical space. Waypoints can be created on a computer-mapping program and uploaded to the GPS receiver or entered manually on the device as a pair of coordinates. Most GPS receivers allow the user to assign a name to each waypoint. As explained later, waypoints are one of the most important concepts to effectively use GPS for wilderness navigation.
In GPS navigation, a "route" is usually defined as a series of two or more waypoints. To follow a route, the GPS user navigates to the nearest waypoint, then to the next one, in turn, until the destination is reached. I have found following GPS Routes impractical when traveling by foot in the backcountry.
Tracklogs provide a travel history by automatically recording a series of points that represent a GPS receiver’s position at a particular time. Like a waypoint, these track points include values for longitude and latitude and sometimes elevation. In addition, track points include the time that they were recorded. Along with waypoint locations and current position, tracklogs are displayed on the map screen of a GPS receiver. Tracklogs are most useful after a wilderness venture. By downloading tracklogs to a computer with a digital mapping program, a visible record of exactly where the GPS receiver (and its user) has been is created.
Digital mapping programs with GPS interface capabilities are essential in using GPS effectively for wilderness navigation. During the planning stage the geographic imagery provided by these programs is extremely helpful for route planning. Creating waypoints in these programs is far easier than entering them manually. Digital mapping programs can also generate custom maps and provide an important platform to view tracklogs, archive data, and share routes.
Basecamp is a Garmin product designed specifically for Garmin GPS receivers. Basecamp is a huge improvement over older versions of Garmin’s Mapsource. Most notable are the improvements in imagery. Topo Series map data is now available in 1:24,000 scale for 49 states and 1:100,000 scale for all 50 states including Alaska. The list of topographic maps available for many international locals is impressive and growing. Additional benefits include the ability to upload base maps and imagery to GPS receivers and the ability to view and archive detailed tracklog data, including direction and speed. Basecamp is reasonably priced and Mac compatible.
For the last 10 years National Geographic Topo! has been my program of choice for planning domestic adventures. Topo! brilliantly uses 1:100,000 and 1:24,000 USGS topographic maps for its imagery in 49 states and 1:52,000 and 1:250,000 USGS topographic maps for Alaska. Using familiar USGS map imagery makes viewing and generating maps easy. Topo! is a popular program used by many adventurers, thus allowing simple file sharing and trip planning. Topo! is Mac compatible.
Previously Memory Map was the only mapping interface option that used topographic map data for many international locations, including Canada and Mexico. Unfortunately, the program is cumbersome and not Mac compatible.
Google Earth is a great and free option for route planning, generating waypoints, archiving data, and sharing trip information. While the 3-D satellite imagery is excellent, Google could take it to the next level by incorporating a terrain layer like it does in Google Maps. Even better would be developing a true topographic map layer. The main limitation with Google Earth is the need for a high-speed Internet connection. Google Earth imagery can be cached on a computer using a technique called Google Earth Cache.
One of the great advantages of trip and route planning trips using digital mapping software is the ease of accurately estimating travel distance and elevation gain and loss. To factor in map error I add 10% to distances plotted on 1:24,000 maps and 20% to distances plotted on 1:100,000 maps. While not scientifically proven I, and others, have found this to be fairly accurate.
Waypoint navigation can be defined as the process of utilizing predetermined sets of geodetic coordinates for route finding and orienteering. This is typically done with a GPS receiver that has been preprogrammed with a series of relevant waypoints.
In geography an absolute location requires a specific set of geodetic coordinates, whereas, a relative location refers to the position of a point in relation to another point. A reference point ideally has an absolute location. Waypoints by definition have an absolute location and are ideal reference points for determining relative location.
By preprogramming a GPS receiver with waypoints (or reference points) it becomes easy in the backcountry to navigate based on relative location. Generating these waypoints is easily accomplished with a computer and one of the digital mapping programs previously discussed. While tedious, it is possible to generate waypoints without a computer by extracting coordinates from a printed map and manually entering them in a GPS receiver.
When planning a trip or route with a digital mapping program I typically create waypoints for important landmarks, including trailheads, summits, structures, passes, rapids, trail junctions, river confluences, etc. These waypoints are then uploaded to my GPS receiver and are included on any printed maps created for the trip. Ideally, the waypoints’ names correspond with the geographic feature they represent.
Most GPS receivers include a function that generates a list of the nearest waypoints, often with distance and direction. This is an extremely helpful function for waypoint navigation that allows a navigator to rapidly triangulate a relative location. For more accurate information on relative location to a specific waypoint, the GOTO function (common to most GPS receivers) is used. The GOTO function determines the exact distance and direction to the referenced waypoint. Coupled with a printed map, it becomes easy for the navigator to determine an accurate location based on the relationship to the waypoint(s).
When following a linear geographic feature, like a trail or river, only one reference waypoint is needed. It is not necessary to triangulate your position. Simply identify the point on the trail or river that is the calculated distance from the chosen reference point.
The compass direction provided by a GPS receiver can, at first, be confusing. This is especially true for navigators accustomed to following a bearing with a magnetic compass. Unlike a magnetic compass, a GPS receiver typically determines the direction displayed on the screen based on the direction of travel, not the orientation of the device itself. If unsure of the compass direction hold the GPS receiver with the navigation page in view and walk a straight line. The GPS will quickly determine and display both true north and your direction of travel.
Traditional orienteering skills can enhance GPS navigation. While GPS is highly efficient for determining a compass bearing, a traditional compass is better for shooting and following a bearing. Monitoring the GPS to ensure the distance to a desired waypoint is decreasing ensures the navigator is following the bearing correctly. I often use this technique on the Colorado Plataea to locate the specific entrance to a slot canyon that I previously entered as a waypoint.
The navigation pages on GPS receivers include other helpful information, including speed and distance. Especially on river trips, I find it both entertaining and informative to note the rate of speed. Knowing the rate of speed and the distance to a location (marked as a waypoint) allows backcountry travelers to make better-informed decisions, such as choosing where to camp.
Handheld GPS receivers typically have a battery life of approximately 20 hours. It is impractical on extended trips to leave a GPS running for the entire time — the weight and expense would be absurd. Instead power on the GPS only when route-finding decisions require it.
I prefer GPS receivers that use standard AAA and AA batteries – the same batteries required by my other electronics like my headlamp or avalanche transceiver. In the backcountry opportunities to recharge electronic devices are rare.
On winter trips and longer trips where every gram matters I prefer lithium batteries. Lithium batteries are not only lighter, they also last longer, especially in the cold. I believe lithium batteries are worthy of the additional expense.
Always remember to never fully depend on anything that requires a battery. Carry a compass and a printed map, and know how to use them. And may the Force be with you.