GPS: The Shortest Distance Between Two Points is NOT a Straight Line

One of the most hotly debated discussions on any cross country discussion board (including our own) is the accuracy of cross country courses.  This article will not attempt to dive into that debate or the exact method you should use to properly measure a course. We will debunk one thing and one thing only: why GPS readings have no place in the discussion.

Any time this discussion starts someone will undoubtedly post about what their Garmin read--or even what their entire team's watches recorded.  The thought being that if you take the average of those readings you should get a good idea of the true length.  Not true.  Even worse, some meet directors now rely on their consumer-grade GPS watch as the final measure.

There is a general misinterpretation of what a GPS device is.  People have a picture in their mind that it is like an odometer or a traditional wheel.  The image being of a consistent and continuous line being drawn along the path that you run.

The reality is that a GPS is a series of dots.  The watch on your wrist is constantly pinging satellites above to determine the point where you are currently located.  To measure distance it simply plays connect-the-dots and adds up the total difference between those points.

There are multiple problems.  First of all, occasionally some of these check-ins are missed for various technical, environmental, or atmospheric reasons.  That means there is more time/distance between those dots.  The GPS device simply waits for the next time it successfully gets a point from the satellite and then connects the dots.  But the further apart the points, the less precise it is to your actual path.

The second (and bigger) problem is the accuracy of those dots.  Garmin's web site itself states the following: "Garmin® GPS receivers are accurate to within 15 meters on average." 

So you say, "psh! big deal! 15 meters!".  No, my friends, that is 15 meters PER POINT.  So the result is that your GPS can read your path as weaving 15 meters in either direction, when you are actually moving in a straight line (see the illustration in this article to get a visual of this effect).  The very latest models may have better accuracy--between 3 and 7 meters--but still yet they create a similar effect (just to a lesser degree).

The result is that a consumer-grade GPS device will consistently read significantly longer than the actual distance.  GPS is a good place to start.  It is a good guide.  And it is a good training tool.  But when you are trying to do official course measure it is not accurate enough to rely on.

So we can debate all we want about lengths or proper wheeling techniques.  But, please, don't insert your GPS reading into that discussion.


More Information about How GPS Works on Garmin's Web Site...