The Race is a 5k but My GPS Only Says 2.99 Miles!! - Tim Price

Posted by Christian Fyfe on

It has happened to all of us…you finish a race in what we thought was a P.R. only to look at your GPS and see that it says your 5k was only 2.99 miles or that the marathon you just finished was actually only 25.87 miles. At this point you are second guessing yourself, the running gods, the head-honchos at USATF (United States of American Track and Field) who certify courses, and just about everyone and everything else.  Well good news…CALM DOWN! you have nothing to worry about.

This link is to an article that explains how the USATF crew measures and certifies a course as well has why you may run a 5k that Mr. GPS says is 3.24 miles instead of 3.1.  I am here to explain why a GPS may come up with a ‘short’ race.

First we need to start with how a GPS works…don’t worry I’m gonna keep it simple.  To start with your watch picks up signals from at least 3 satellites that are thousands of miles (about 12,500 miles!!!) away somewhere up in space orbiting the earth. That in itself to me is freakin amazing.

So once the GPS unit has locked onto satellites (and we have all also done that crazy dance of lifting our arm up, running around, and cursing to try and get a lock) you start to run.  As you run your watch is continually sending and receiving signals to the satellites and recording the latitude and longitude coordinates. As these coordinates are collected a straight line is draw from one, to the next, to the next, and so on until you disconnect from the satellites. In a nut shell that is how a GPS works.

But why was your race short?
Check out this image from Garmin Connect that shows the GPS determined route I ran during a race earlier this year….


If you look at that map I appear to run thru some houses around those sharp corners…of course that isn’t true.  But the GPS doesn’t know there are houses there.  It received the first reading on one side of the corner and did not get another till I had turned the corner and run down the road a little.  And the tracking software did exactly what it was supposed to…connect the dots.

Sharp turns and corners are the big player in short course readings: the software tracks shortest possible distance so by making an abrupt turn you are covering distance the GPS doesn’t record and the tracing software doesn’t map.

Other discrepancies can come from tall buildings, heavily forested areas, high cloud cover, and other conditions that make it hard for your GPS to stay locked onto satellites.  Going in and out of sync with satellites is another reason a course may be ‘short’…your watch cannot collect data it isn’t receiving.  I have also found that if you wait till the last minute to turn your GPS on before a race you may not get a signal.  I have no proof but I think that with hundreds or thousands of other people doing the same the satellites are just overwhelmed and cannot connect with everyone.  My advice is to get set up and ready to go at least 10 minutes before start time.

If you are racing a course that is Certified trust in the credibility and rigid guidelines required for USATF Certification and not in your trusty (or maybe not so trusty) GPS. So go ahead and celebrate that new P.R.!

- Tim Price


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