Navigation: Can You Trust Your GPS?

by Phil Romig Jr


When I started teaching, I asked a wise old professor: “How do I make a dry, boring subject more interesting?” The answer was: “Talk about something else.” That came back to me when I started on this column. Details about accuracy and precision are boring and arcane to most people. All they really want to know is: “Can I trust what my GPS is telling me?”


The short answer is:
1. Locations usually are accurate to about 30 feet, and a good GPS will tell you if it has a problem.
2. Distance traveled can be wrong by 10% or more, and your GPS won’t tell you when there is a problem.


Many people—surveyors, utility workers, farmers, etc.—trust GPS to do their jobs. Others— pilots, search and rescue teams, etc.—trust it with their lives. On the trail, though, the only thing that matters is whether you trust your GPS.


?Can you trust your GPS?
Your answer might be: “That’s a dumb question! Why would I spend several hundred bucks on a device that I can’t trust?” On the other hand, do you trust your pace always to be exactly 5.28 feet, or do you know that it changes with trail conditions? Do you trust your compass to point exactly north, or are you aware that it points toward magnetic north? Do you trust your map to have trails in the right place even though it may be forty years old?


Experienced packers know when and how much to trust their pace, compass or map, and they can take into account things that cause errors. The same principle applies to your GPS receiver.


Used properly, GPS receivers provide more information and are more accurate than other backcountry navigation devices. However:
They require batteries that eventually run down.
They have electronic components that can fail at any time.
The first step toward trusting your GPS is always to carry spare batteries and have a backup (map and compass) that you know how to use. Always!


Last year, an experienced motorcyclist convinced a friend to take a two-week ride in Death Valley. To meet his friend, he would have to ride alone the first two days, so his wife and son convinced him to buy a combination GPS receiver and satellite messenger. The morning of the second day, in a remote area, he took a hard fall and broke his leg. He pressed the SOS button on his GPS/messenger, and Search and Rescue arrived within a couple of hours.


Apparently the motorcyclist and his family knew that there would be no cell phone service in many areas and that the dry climate of the desert was well suited to GPS reception. Picking the GPS device best suited for those conditions allowed them to trust that it would work as expected in an emergency.


The second step toward trusting your GPS is to have a solid, general understanding of:
How accurate your GPS locations, tracks, routes and calculations are
How conditions that you encounter on the trail can degrade its accuracy
What you can do to minimize the impact of those conditions


Accuracy vs Precision
At a rest stop on a hike last summer, the digital compass in our GPS gave 154.5 degrees as the bearing of a prominent peak. With our magnetic compass, we estimated the bearing at somewhere between 143 and 147 degrees. The digital reading obviously was more accurate, so we plotted its line of bearing on our map. It showed that we were about 1.5 miles from where we thought we were!


Our mistake was to assume that the digital compass was accurate because its reading was precise to 0.5 degree. With a little experimentation, we discovered that tilting it slightly could change the reading by as much as 30 degrees. For a reading to be accurate, it must be held exactly same way as when it was calibrated, but someone else had calibrated it. The digital compass was ten times more precise but five times less accurate than the magnetic compass. Furthermore, we knew how much to trust the magnetic compass but not the digital compass.


The precision of a device is a measure of how many digits there are in a reading. Accuracy is a measure of how close the reading is to the truth. In the backcountry, the goal is to know the truth—your location—accurately. Don’t be deceived by digital readings with a lot of numbers. Know how much to trust your instruments—how they work, how accurate they are, and what affects their accuracy.


At that same rest stop, the magnetic line of bearing confirmed we were where we thought we were. The fact that dead reckoning, the line of bearing and the map features agreed with the GPS gave us the confidence to use the GPS coordinates as our exact location.


Accuracy of GPS Locations and Tracks
GPS locations are based on radio waves that travel from satellites to the receiver. Anything that the waves pass through will cause errors, including clouds, rain, snow, thunderstorms, atmospheric pressure and tree cover. If mountains, cliffs and buildings are in the way, the waves must travel around them or may be blocked. The best locations are produced by receivers that are in the open in good weather, but good receivers can do reasonably well under less ideal conditions.


A “track” is a point-to-point record of where you have been. Most GPS receivers can use multiple locations to calculate information such as speed and distance travelled. Location errors affect the accuracy of these calculations, but the formulas used by the receiver can cause even larger errors. Some GPS units let you customize the formulas, producing different results from the same locations.


A reader recently told of an experience where three different people carried GPS units on a pack trial. All three walked together on the course. At the end of the course, one receiver said they had walked 8.4 miles; a second showed 9.3 miles, and the third claimed it was over 10 miles. Who’s right, if anyone? How can we reduce those differences?


We’ll do our best to answer these sorts of questions in the next couple of columns. In the meantime, You can improve accuracy by using a GPS receiver that:
Has a good antenna and high sensitivity
Calculates locations using as many channels (satellites) as possible
Combines all channels simultaneously rather than sequentially
Allows you to set the interval between locations when recording tracks
Shows a current estimate of location accuracy and/or satellite locations
Is WAAS (Wide Area Augmentation System) enabled


Use of receivers with these characteristics and all of the right settings will go a long way toward providing location and distance data that you can trust.