Confusion - don't trust equipment!

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2011-05-03 9:41 AM

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Subject: Confusion - don't trust equipment!

Hi,

I did my first *lengthy* run today I say lengthy because my longest run has been 3km before: This is the route we were took (from mapmyrun.com)

 

ITs a 4.65km run.

 

I must admit I was scared as its the furthest but felt when I did it that it went really well .... until I saw my results from my gps runner recorder thingy me bob (never ever had a problem with it) which said I only did 3.39km in 35.48 - I didn't know the length of the route before hand so was actually upset and very very annoyed because I thought my training was coming on a lot better.

However I am thankful that GPS shows a route because this is what it showed:

 

I know you probably can't see on some of it but the gps is all over the place - it must of had a glitch in it because we ran right around the field and all the way down to the roundabout - not to mention we didn't run on the road or go through a massive metal fence!

So actually I'm happy it seems that 4.65 is nearer than the 3.39 - the only problem is I have no idea if the timing was off as well as the gps .......... If it was I haven't got a scooby how long it took me. If it wasn't then I'm in a brilliant mood because its the furthest I've done and on target to where I need to be!

The other problem -which will have to wait till I go out again (I'l test it when I take the dog out) is was it a one off glitch or a major problem thats gong to be ongoing!

2011-05-03 10:29 AM
in reply to: #3479529

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Subject: RE: Confusion - don't trust equipment!

1. Your time should be accurate, unless you accidentally started or stopped the timer early/late then there is no reason your time should be off.

2. The way a GPS tracking device works while moving is that it looks for a satellite lock every few seconds (or some given time interval) and then looks to see if that point can be trusted given how many satellites, triangulation, etc (with relatively high accuracy, mine is set to >90%). When it locks on a point it draws a straight line from that current point to the last point it received and measures that line.

Therefore, if your route has a bunch of turns or is an out and back or there was something blocking satellite signals (clouds, trees, buildings) then the reading won't be very accurate due to the way the measuring is taking place.

2011-05-03 10:34 AM
in reply to: #3479529

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Subject: RE: Confusion - don't trust equipment!

What GPS unit do you own?  I know on the GArmin there is a way to set the sampling rate.

You must have lost signal on the "across the field" trip.  That's way beyond normal error.

2011-05-03 11:03 AM
in reply to: #3479529

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Subject: RE: Confusion - don't trust equipment!

I'd go with the MapMyRun distance.

I have a new Garmin 310Xt. In this past weekends marathon. It's overall time was correct with the official time. But the Garmin recorded 26.65 mile for a certified 26.2 mi course. Given there is a bib of bob and weave and I didn't cut all the turns as tight as could be. I doubt I added almost a half mile.

2011-05-03 12:00 PM
in reply to: #3479722

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Subject: RE: Confusion - don't trust equipment!
halfmarathondon - 2011-05-03 11:03 AM

I'd go with the MapMyRun distance.

I have a new Garmin 310Xt. In this past weekends marathon. It's overall time was correct with the official time. But the Garmin recorded 26.65 mile for a certified 26.2 mi course. Given there is a bib of bob and weave and I didn't cut all the turns as tight as could be. I doubt I added almost a half mile.

I'd believe you were off by that much. The little differences add up when you don't run the full tangents. Not to mention the inherent error with GPS.

2011-05-03 12:32 PM
in reply to: #3479722

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Subject: RE: Confusion - don't trust equipment!
halfmarathondon - 2011-05-03 12:03 PM

I'd go with the MapMyRun distance.

I have a new Garmin 310Xt. In this past weekends marathon. It's overall time was correct with the official time. But the Garmin recorded 26.65 mile for a certified 26.2 mi course. Given there is a bib of bob and weave and I didn't cut all the turns as tight as could be. I doubt I added almost a half mile.



A) Assuming it was a certified course, then it is automatically longer than 26.2 miles (Short Course Prevention Factor).

B) Unless you follow the EXACT route taken by the course measurer, you will not get the same distance. And if the person doing the measuring was doing so for certification, that person took every tangent for every turn.

C) Garmin has at best, a radius of about 10 meters (variable, of course). That means that each point it plots is actually somewhere within, at best, 10 meters of the actual physical location. Which means that there is a potential to add at least 20 meters when two points are plotted. And depending on how many plots you take and how many turns you have, you increase your chances for getting weird numbers. If you doubt this, wear the Garmin while running on a track, and look at the plot afterwards. Tracks make GPS go berserk.

As for MapyMyRun and its accuracy, depending on how close you zoom in, you could get some level of discrepancy. Although, I would say that in this case, MapMyRun is far more accurate than Garmin.

2011-05-03 12:40 PM
in reply to: #3479529

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Subject: RE: Confusion - don't trust equipment!
2011-05-03 1:09 PM
in reply to: #3479529

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Subject: RE: Confusion - don't trust equipment!

I use MiCoach which is usually very good - and I find very accurate. I think mabye it was having an off day lol if computers can have off days!

I'm going to use the 4.65km because that seems to be the distance we ran according to map my run plus it gives me a boost :D

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