That Goofy GPS

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I’ve been asked by numerous folks who are members of my running meetup group about GPS watches and how they sometimes seem goofy.  Ever have a problem where it seems inaccurate?  Ran the same route numerous times but get different distances?  Well, you’re not alone.

There are multiple things going on here, but basically, you have to realize that your GPS device is inaccurate.

Here’s an interesting tidbit. If you check out USATF instructions on how to measure a “certified course”, you’ll notice it has a pretty strong stance about using GPS devices, because they know how inaccurate they can be.  ­

But some of you may be saying, “Wait a sec… I’ve seen land surveyors using GPS devices.  What gives?”  So how do land surveyors get accurate readings with GPS? First off, they require two GPS measurement devices (much more accurate than our wrist-based devices and much larger in size) used in conjunction with a starting known position. All measurements are made from that known starting position, one that has been accurately verified and measured, usually marked permanently.

Let’s take a look at your device and where it falls short. Consumer GPS devices can be inaccurate for numerous reasons. Here are but a few:

  1. Atmospheric conditions – yep. the usual stuff, rain, temperature (density), humidity, precipitation, among other effects of the ionosphere and troposphere (too much to explain here, but I can expand on it if you want).
  2. Multipath issues – these result when the direct path to your receiver is blocked (by your body, your house, roof, trees, mountains, buildings, etc) and the signal from the satellite is REFLECTED by some object. The reflecting surface could be buildings, mountains, the ground, or other radio emitting objects. In the case of running on the trail, running in and out of valleys can also cause multipath issues. When you have a multipath problem, it causes the GPS signals to travel FURTHER than expected to get to your receiver. The result is your GPS miscalculates its position. This can cause the biggest variety of error because the signals may have traveled anywhere from feet to miles further than expected (as opposed to direct line of sight which it’s expecting).
  3. Signal Noise – Noise results from static or interference from something near the receiver or something on the same frequency.
  4. Problems with satellite orbits – There can be inaccuracies in the measurement of the gps satellites’ orbits. These are considered “ephemeris errors”. Obviously, we need to know the accurate measurement of the satellites’’ orbit in order to accurately measure distances. These can be fixed with accurate satellite information and use of more satellites.
  5. Clock drift – This is where the clock on the GPS unit is not set to the same time as the time on the satellites. This is corrected, usually when you turn on your GPS, when it sets the time via the time of the satellites. However, the clock on the GPS unit is not as accurate as the ones on the satellites (which using atomic time). The longer you have your GPS on, the greater this may drift, especially if it only updates on initial power on.

There are a bunch more, but these are the big ones.

With all of the reasons that can cause error, it sums up to these possibility of these amounts:

  • Tropospheric effects ± 0.5 meter
  • Ionospheric effects ± 5 meters
  • Multipath effect ± 1 meter
  • Noise and Calculation rounding errors ± 1 meter
  • Satellite orbits ± 2.5 meter
  • Clock errors of the satellites’ clocks ± 2 meter

Altogether this sums up to an error of ± 15 meters at any given point of time. Couple that with an hour-long run and you can have measurement and pacing errors.

So what can you do about it? These problems above? Not much. HOWEVER, there are two “fixes” you CAN implement to try and get a better reading.

Fix Number 1: Keep the software on the device up to date. Periodically, Garmin and other manufacturers will release updates to the software that runs the GPS units. Sometimes these updates include algorithms that can improve accuracy and get rid of glitches in the code. How to update it depends on the model and manufacturer. Check their website for info on how.

Fix Number 2: is not really a fix, but rather a change in configuration.
Most folks use their GPS device with the configuration that it has out of the box. This would be fine for road runners, as they tend to run in a relative straight line (for the most part). But for us trail runners, the default configuration can cause errors.

Here’s how:
Out of the box, most GPS devices measure distance by taking a sampling at specific intervals. Garmin devices use a method called ‘Smart Recording” that takes a measurement anytime that you change direction, speed or heart rate. By only taking measurements at these events, it conserves battery life, IF you were running on the road. Unfortunately, that’s all the time on the trail. The problem that I have seen is that it gets quite confused with switchbacks, the constants speed changes with the terrain and to make matters worse, you throw in the signal multipathing issue.

So in order to FORCE it to give me a more accurate reading, I have found it best to set it to “Record Every Second”. (Just go into your Garmin’s Data Recording setting and change it there. I get more accurate readings, but you still have to take them with a pinch of salt.

To summarize, should you trash your device? No. It can give you great information about heart rate and other valuable data. Plus, even with incorrect distance and pacing, running the same course on subsequent workouts can give you information from each run to compare and to use for measuring any type of improvement in fitness. However, don’t take the distance and pace readings as being completely accurate. As a trail runner and mountain biker, the trails can magnify the inherent problems that GPS devices exhibit. Keep them and use them, but keep in mind they may be off a bit. And above all, don’t gripe at the race staff at your next trail race at how the course distances are wrong. If the race director is any good, he measured it with a calibrated odometer on a mountain bike, not a GPS.

Happy Trails