March 30, 2020

Why GPS fails for Hikers

Filed under: Hiking — Tags: , , — geezerwriter @ 7:34 pm

Now that everyone and her Aunt Mildred carry a GPS device everywhere they go, the scene at the end of our (temporarily suspended) group hikes sounds a bit like this:

  • “I got 8.3 miles. What did you get?”
  • “I got 8.7 miles. And you?”
  • “7.9”
  • “OK, let’s round off to 9.”

Why are these sophisticated devices returning such disparate results? (I take a lot of flak because I’m always, I mean ALWAYS, the party pooper with the “7.9”.)

I think I can explain in this posting (which is long but has lots of pictures and very little Math) a lot of what is going on with some tracks that I collected on a short, steep hike last Thursday with two other senior hikers. We hiked slowly (because I am extremely old); because of the Coronavirus emergency we were careful to maintain a good bit of separation and we stopped several times at wide spots in the trail to chat because it was hard to converse when one of my companions was about 15 feet behind me! (You might be able to guess what we talked about 🙂 )

My usual electronic companion, a handheld Garmin GPS, had been forgotten and left at home so I started up the Polar Flow app on my new iPhone. It reads my heart rate from a photoelectric sensor on my arm and takes position data from the phone’s GPS to construct a detailed picture of my hike – it hasn’t impressed me as very accurate but I do love Data and it would have to do.

The first time we paused to solve all the world’s problems I had the bright of idea of also starting up the GaisGPS app. It would be interrogating the same iPhone GPS data as the Polar Flow so it might be interesting to compare them.

I ended up with a bunch of tracks. When I got home I exported them as GPX files and uploaded them all to Garmin’s BaseCamp software, where I could plot them all on the same map and select portions of interest – I selected out just the portion from the place where I started Gaia to the top of the trail. And I dug out an old Garmin track of the same trail.

Pine & Cedar Lakes Trail – uphill portion

This first map shows the entire hike. All three tracks are present but only my old Garmin track is highlighted (in green); the iPhone tracks peek out (in purple) here and there. The place where I started Gaia shows as a little purple blob at the 800′ contour line, and the rest of the pictures will cover from that point to the south end (at the left).

All tracks

This map has all three tracks highlighted but we can only see two because the iPhone tracks are right on top of each other. The tracks look quite different but please notice that they have the same overall shape. The phone tracks are a lot kinkier – I think the green track looks most like the trail as I remember it. Some numbers that are pretty interesting:

  • Polar: 2.2 miles – 4,264 readings
  • Gaia: 1.5 miles – 481 readings
  • Garmin: 1.2 miles – 211 readings

Now I’ll focus on the Polar Flow track, which is in magenta

Polar Flow track

Notice that the magenta track has several blobs or fat spots, which happen to lie near the 800, 1200 and 1600 foot contours. These are the major places where we stopped and had a chat. I’ll zoom in on the one at the top of the trail, which is the biggest:

20 minute chat at the top of the trail

What happens is that the Polar app cannot tell that we are stopped because the phone’s GPS keeps sending it a location every second and each of those locations is slightly different from the last, because of the inherent and unavoidable errors in the very complex and sophisticated process of divining our position from timing radio transmissions from a bunch of satellites hundreds of miles away. The GPS cannot tell directly if we are moving – it only has an estimate of our position at each point in time.

In fact, the app has recorded about 230 readings during the roughly 240 seconds that we were standing and talking. These readings are all pretty close together, thanks to the pretty good accuracy of the phone’s GPS, but the Polar app does not see anything suspicious about hundreds of readings within a few feet of each other and no sense of progress. All of these spurious little distances add up to roughly a half a mile! And after deleting this blob and the other two, the track loses almost three-quarters of a mile,. So the original track length was high by 45% – and that’s just the worst stuff.

Now the difference this and the Gaia track is that the Gaia app doesn’t accept blindly all the data proffered but the GPS, but applies its own algorithm in an attempt to filter out some of this noise. And it does a pretty good job but it still shows blobs of readings at those same three places – removing them brings the numbers to:

  • Polar: 1.5 miles – 2362 readings
  • Gaia: 1.3 miles – 327 readings
  • Garmin: 1.2 miles – 211 readings
Central portion of hike

Looking at this enlargement of the central portion of the hike, one feature is really striking:

The (faint green) Garmin track is smooth-looking and free of those blobs. The phone tracks have all sorts of wiggles and jukes that just ain’t there in real life, including an actual loop where the trail has just a tight pair of switchbacks. (Very few trails have loops.)

I think it is pretty clear that all those extra points (the ads will call it “greater precision”) in the phone tracks do not simply fail to improve the track but actually make it much less accurate – the Garmin track is vastly superior to the others.


  • Most phone based GPS programs are junk for serious hiking – they don’t even produce very good results when walking in town. (The Polar company makes heart rate monitors and is focussed on calorie consumption and doesn’t pretend to be an orienteering program.) You’d be better off with a well-calibrated pedometer.
  • The GaiaGPS app is much superior. It’s tracks are still jagged and strange looking but the overall distance measure is only a little bit high. But this is true IF AND ONLY IF you remember to pause the tracking manually whenever you stop for even a couple of minutes so the blobs won’t have a chance to form. This is a deal-breaker for me personally because I’m just too forgetful.
  • The Garmin GPS units (and probably other brands) can give excellent tracks and distances but NOT if you use them as they come from the factory. They come set up for use in vehicles but you can probably rejigger the settings to get good results when hiking.

This state of affairs is quite unfortunate and unnecessary. Modern phones have enormous processing power, far more than my 5-year-old GPS handheld, and could certainly be programmed to filter out most of the garbage. Gaia has made an excellent start in this direction and I plan to try to get their attention and see if they can do even better. (Several years ago, Garmin gave me the runaround for 6 months and ultimately did nothing. I doubt if Polar would even care.)

I’d like to give some more tips on how to get better GPS results but this post has already gotten way too long. Stay safe and hike responsibly.


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