News - 26th July 2018
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Measuring our 5k parkruns

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This week Lawrence, our Event Director at Meadowvale, explains the nuances of GPS and how it differs from our measured 5k courses.

 

A parkrun course is a distance of 5km. Meadowvale parkrun is a 3 lap course around a lake with a slight dogleg added to the end to make up the distance to a full 5kms.

 

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A measuring wheel was used to determine the distance of 1 lap. It was measured twice in opposite directions (4 times total) to get an average distance.

 

So why does my GPS tracking device measure differently?

 

To answer this, it is helpful to understand how the GPS system works.  For this explanation, a watch will refer to any tracking device that people use for the activity ( watch/phone/app etc).

The GPS system currently has 31 active satellites orbiting about 20000km from the earth’s surface and each one makes two orbits per day. The orbits are designed so that there are always 6 satellites in view, from most places on the earth.

 

GPS uses a lot of complex technology, but the concept is simple.

The watch gets a signal from each GPS satellite. The satellites transmit the exact time the signals are sent. By subtracting the time the signal was transmitted from the time it was received, the watch can tell how far it is from each satellite. The watch also knows the exact position in the sky of the satellites at the moment they sent their signals. So given the travel time of the GPS signals from three satellites and their exact position in the sky, the watch can determine your position in three dimensions – east, north and altitude.

 

The more satellites the watch can see, the more accurately your position is calculated.

 

In a perfect scenario, you would be running on a cloudless day in an open area. This is because the satellite signal can be affected by clouds and can also reflect off buildings, trees and other objects to create slight errors that the watch picks up. This contributes to a watch having a real world accuracy of about 3-10 metres of your true location.

 

The watch also doesn’t actually determine your position continuously. It does it every few seconds (usually 4 or 5). So if you start at point A and 5 seconds later are at point B. The watch calculates the distance between the 2 points to know how far you travelled. The problem with this is that you quite likely did not move in a straight line and covered slightly more distance. An extreme example would be that you run in an “s” shape for those 5 seconds, but the watch would still only see it as a straight line. Therefore you may actually run 10 metres, but the watch measures it as 9.5 metres.

 

This snapshot of a bend in the course at a recent Meadowvale parkrun illustrates this perfectly. It also shows that although I ran the same path 3 times, each time the watch recorded a slight different route.

 

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In summary, when you look at all the technology involved. I don’t wish a GPS watch was more accurate, I am actually impressed that it measures a parkrun 5km course as well as it does.

 

Lawrence Jeffery

Meadowvale parkrun Event Director

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