The returning runner

Act 19 – Why does my GPS/iWatch give a different distance every time I run the same course?

Technical Contributor – Jerry Juarez, PLS

You have probably noticed this phenomenon. You run the same course and get a different distance – sometimes varying by hundredths and sometimes quite a bit more. Maybe you swung a bit further out going around a turn? Likely not – the cause is most probably just the way that your mobile device is calculating your distance. An obvious example of this can be illustrated by starting your device on an outdoor workout and then standing in one spot. If you keep the “workout” going, your device will continue to add small amounts of distance even though you are stationary. Modern mobile devices use Global Positioning Satellite (GPS) location information to calculate your position and, mathematically, distance covered. A couple of issues affect the accuracy of these calculations:

·       The accuracy of the GPS chip varies considerably based on the time of day and the physical conditions associated with your location.

·       Your mobile device collects information at discrete intervals – typically 3 seconds.

Accuracy Issues

Your GPS chip relies on the strength of satellite signals received and the number of satellites within “range” to provide better triangulation of your position. Professional surveyors can check satellite positions at various times of the day to help provide information on the accuracy that they can obtain. Additionally, steep terrain and dense vegetation can block satellite signals. If you are running in a steep canyon or heavily forested area, the accuracy of your position could be quite a ways off. In areas where terrain substantially blocks satellite signals, your position accuracy can be off by 100 feet or more.

So why do I get different distances?

The reasons for inaccuracy in GPS positions give us some clues as to why you might get a different distance on the same route:

·       If you run at a different time of the day, the satellites will have different positions and signal strength.

·       If you run at a different time of the year, the vegetation obstruction can be quite a bit different (less foliage in the fall for example).

·       If you run at a different speed, you may get a different distance reading. Remember, the GPS chip collects position data at 3 second intervals. On your best days, the GPS is more likely to miss a portion of switchbacks and understate your distance!

·       The most common and largest errors are typically a significant understating of distance on very windy trails in steep terrain or areas with dense tree canopy cover.

·       It is possible for your GPS to overstate the distance on a very straight route. Running along a bike lane marking on a straight road, for example, you can run a very straight line. However, your GPS location may be bouncing back and forth on either side of your actual route – particularly if there is steep terrain (a narrow, straight canyon for example) or a dense tree canopy.

Cheers – enjoy the fact that the largest errors are typically understating your distance covered! I always log the longest distance my GPS has provided when running the same route.

Graphic: Example of potential GPS distance measurement “cutting off” switch backs on the Dry Pond loop outside of Reno, Nevada.