When GPS Doesn't Work

The Global Positioning System (GPS) is an incredible tool for the outdoor adventurer. But like any tool, GPS has limitations. As a hiking guide author, I've had readers complain that the trail distances in some of my books are too short. When I ask how they know, the answer is always "I measured it with my GPS."

You can't. Why? Because GPS isn't accurate enough. GPS receivers measure distance (and speed) by taking a series of position fixes as you travel and measuring the distance between them. Trail GPS receivers all have the same accuracy, 10 meters or 33 feet.  This means that as you walk, at about two or three miles an hour, the GPS position fixes are not at your exact location, but up to 33 feet away (actually, the error can be even more when the sky is partially obscured or the satellite geometry is poor.)

So, instead of position fixes that are like a series of dots along your path, they are randomly scattered around your route of travel. GPS measures the distance between each of the these position fixes and comes up with a total distance that is always greater than the actual length of the trail. How much? My field tests have shown errors as much as 50%. In other words, GPS calculates the length of a 10-mile trail as 15 miles. More typical errors are in the range of 5 to 20%.

A quick way to demonstrate the effect of these cumulative errors is to turn your GPS on, place it where it can get a position fix, and then erase the track log and clear the data on the odometer page. Leave it on for a few hours without moving it and check the the odometer page to see how far the receiver thinks it's traveled, and how fast.

How can you accurately measure trail distance? One way is with differential GPS (DGPS), which uses a GPS receiver and differential transmitter placed on a known surveyed point. The DGPS unit compares the difference between its actual location and the GPS computed location and broadcasts the correction to DGPS-equipped receivers within range. DGPS is not practical for recreational hikers and is used mainly by surveyors and scientists doing field work.

The most accurate practical method to measure trail distance is the one used by the U.S. Forest Service and other land management agencies: Roll a trail wheel over the trail. Trail wheels work just like a cyclometer on a bicycle. Knowing the circumference of the wheel, a counter adds up the total distance rolled. Recreational hikers are not going to bother with this method, though mountain bikers can certainly use it on trails open to bicycles. That's how I measure trails for my mountain biking guidebooks.

The next most accurate method is to trace the trail on a large scale topographic map. In the pre-computer, pre-GPS days, that was done with a map wheel on a paper map. The results depend on how carefully you were willing to follow the twists and turns of the trail. I found this method to be about 10% short of the trail distance as measured with a cyclometer. Now that we have digital topo maps, we can trace the trail on the screen using the mapping program's freehand route tool. My tests, compared with cyclometer measurements, have shown this method to be about 2% short.

This discussion applies to walking speeds. If you are moving faster, such as riding a bike or paddling a boat, the 33-foot errors are not as large a factor and the amount of error is much less.

In summary, the only accurate way to use a trail GPS is measure trail distance is to record a track in the field, import the GPS track to your digital topo map, and then trace over the GPS track with the program's freehand route tool. Don't try to measure the distance of the imported GPS track in your digital map program. The position errors are present in the downloaded track, even though you can't zoom in enough to see them.