Imagine for a moment that you just completed an epic hike. Your handy dandy GPS application reported your distance at a whopping 17 miles. You screenshot your app, and post is to social media. Within minutes, you get some know-it-all saying, “Well, actually, that hike is only 15 miles.”
Punked! Either that know-it-all on social media is a know-it-all, or your GPS app has just done you dirty. The truth? The know-it-all is right.
First a quick primer on how GPS works. Phone applications and the dedicated GPS units determine your location by triangulating it between three to five satellites. If you’re moving, you get an approximate (but ultimately not fully accurate) idea of where you are on the planet, which your GPS unit extrapolates and then overlays on a map. If you look at your GPS track overlayed against a satellite image, the GPS track is almost never 100% aligned with the trail. This discrepancy accounts for a small percentage of erroneous distance, but it doesn’t account for the glaring discrepancies that often pop up.
Most people leave their trackers running at all times, which doesn’t just include time on the trail. It includes times where you go off-trail (bathroom stops). It includes time where you might have backtracked or walked additional distance for one reason or another. Most importantly, it includes time when you’ve been stopped. Check the Gaia screenshot I posted below; this shows my track from the 2 minutes I left it running when I arrived at Parker Lake before I shut off my tracker. Can you see the big squiggle?
When you’re stationary, the tracker continues to triangulate your position. But because the satellites are always in motion, the tracker records your location in a slightly different spot. This creates what I call “GPS spaghetti,” as it creates a tangled line that resembles a plate of pasta. The GPS tracker considers this bouncing around as movement, and it adds that extra distance, which I call “phantom distance” to your track. The longer you sit in one spot, the more phantom distance gets added to your GPS track, which inflates the overall distance. This inflated distance leaves you with the impression that you hiked farther than you actually did.
There are some technical ways to correct phantom distance, and guide book authors will use those methods to “clean up” the phantom distance and produce a GPS track that’s closer to true distance. However, no GPS track will ever be 100% accurate based on how they’re created (see the above misalignment picture for example). The only fool-proof way to measure a trail’s distance is to use mechanical methods, such as a very long chain marked with distance intervals.
So, next time your GPS track comes out different from the commonly accepted distance, and you get the feedback that your distance is off, you will know why. GPS tracks are inaccurate by nature, and your distance gets inflated even when you’re sitting still. There’s no sure-fire way to know how far you’ve walked, so you basically have to live with a rough estimate.