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Old 07-04-07, 02:31 PM   #1
Snicklefritz
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Garmin forerunnin 305 - distance mismatch issue?

I normally post in the road forum but thought my question might be more appropriate here.
any of you tri people use the forerunner and notice issues with the distance being off?

I just started running this year and have done some footraces and du's. I've noticed on several
occasions that the distance my GPS says I did is different from the measured race course. I'm sure
there is always going to be some mismatch because the path I run may not be exactly where they
measured the course. But 1/2 mile off over a 10k or 12k course? That seems a bit weird. The last
two races I did were 10k and 12k and in both cases the GPS said I ran 0.4 to 0.5 more than what the
course was supposed to have been.

If it's a certified course, how could the GPS be that far off? I can't imagine that following different line would produce that much of a difference. So what's up with this?

I was trying to analyze some data from today's race when I noticed that it said I did 6.7 miles instead of 6.2 miles (actual course). So now when I go in and try to figure out what pace I was doing for a given segment, I am sure the numbers are going to be off.
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Old 07-04-07, 03:03 PM   #2
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Every year that I run the Valpo Mini Marathon (13.1 miles) my forerunner 201 always says 13.4 miles. At least it's consistant!
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Old 07-04-07, 03:28 PM   #3
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I was thinking the same thing as Valpo's comment about consistency. Does the Garmin show the same distance every time over the same course? I've always assumed that mine was accurate due to GPS tracking but maybe not. I guess I'll need to give it a try.
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Old 07-04-07, 03:32 PM   #4
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Quote:
Originally Posted by cjbruin
I was thinking the same thing as Valpo's comment about consistency. Does the Garmin show the same distance every time over the same course? I've always assumed that mine was accurate due to GPS tracking but maybe not. I guess I'll need to give it a try.
I haven't tested mine over the same courses for these 10k and 12k. Most have special road closures so you can't go out and do this. However, I'm going to go out sometime this week or next with my bike and compare the data from the 305 and my powertap to see what difference I get over 5k to 12k distances and see what happens. I'll post the data here if people are curious.

It just seems strange that it's off by the same factor every time.
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Old 07-04-07, 04:14 PM   #5
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I'm off to a July 4th BBQ in a bit but have some numbers.

I looked at my powertap for an interval session I did last week and for a certain one it said I did 5.544 miles. For the same interval, the GPS said I did 5.532 miles. There is only a very small % error introduced from cursor positioning in the cyclingpeaks program.

This is MUCH less than the error I have seen in the footraces. Not sure if it is related to the speed I am moving at and the GPS's ability to track my movement. But you'd think if I were moving slower it would have an easier time of it. Anyways, most of my cycling routes show similar distance matching. Interestingly, I tend to ride in places where it is very open, not a lot of tree cover, etc. The foot races have been a different story.

Do you think the GPS gets screwed up somehow if you run in a shady area with a lot of tree cover?


I'm still not sure why I see good matching between the bike and powertap and a big discrepancy while running.
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Old 07-05-07, 05:56 AM   #6
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possible the gps picks up the up and down movements while running which dont occur on a bike and that causes slightly higher readings?
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Old 07-05-07, 06:08 AM   #7
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I have a friend with a Forerunner but no personal experience. A few things:

-Tree cover can affect it. He has lost signal before and the distance shows short. Also it has gotten some weird readings and just shown a completely wrong number.

-Buildings and downtown type areas affect signal similar to treed areas.

-The turn on process can affect it as well. With some research on the web we have found a bunch of people having similar issues. The biggest fix is satellite connectivity. People have found that sometimes the Forerunner doesn't get a good signal quickly and it can affect the reading and distance. Try turning the Forerunner on and set it somewhere very open (sidewalk, driveway, in a window, etc) for at least 5 minutes without moving it or anything. We are thinking possibly it acquires a couple satellite signals if it is just turned on but acquires more if left out as mentioned.

With mainly the turn-on process described above my friends Forerunner has been a lot more accurate. Races have been showing very low error percentages like you are seeing on your bike rides. You may try a few runs in the open as well if you are doing a lot of treed runs.

Hope this helps.
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Old 07-05-07, 07:31 AM   #8
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http://www.gmap-pedometer.com/

Use this and see what you get, its a great tool.
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Old 07-06-07, 03:10 PM   #9
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I've got the Edge -- the biking version of the Forerunner -- and its distances are consistent. Consistent is obviously not the same as accurate, but I have no reason to doubt its accuracy. In fact, if I ride up one side of a narrow street, and down the other side on my return trip, the map of my ride clearly shows the difference. If the GPS is registering that my "out" path is five to ten meters to the left/right of my "back" path, I would think that it couldn't be screwing up distance by too much.
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Old 07-06-07, 04:29 PM   #10
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Its quite possible that the courses that you've been racing at are really off! I own a Garmin, and I believe it is very accurate. There have been a couple of races I've been at this year where the Garmin has measured a shorter course than that it was supposed to be. And I hardly ever race (so there haven't been that many I actually measure).

For example, the Flint Hills Triathlon in Manhattan Kansas. Supposed to be "certified" (indeed, its an amateur worlds qualifier). Anyway, I finished it and my posted pace/finishing run time was 6:13/38:40. The relevancy of the time here is that I was like blown away "no way, was I that fast??" I look at the summary in "History" and the Garmin had me at a 6:27 or so pace. So I bug the race director. He goes out and measures the run course 2 more times (with a measuring wheel and a GPS), and it turns out to be 5.99 miles. That's enough to make a big difference.

Of course I'm somewhat happy in this case, because the distance mismatch was favorable for me... then again, though, I'll always have that hanging over me ("yes, officially you ran a 38:40 10k, but what would it really have been...?"). And I obsessed over it for days. It was my only Olympic distance triathlon and I really wanted to know what sort of times I can reasonably expect at my next race.

And another case about 2 weeks ago at this 5k, the distance was so badly marked that it was closer to 2.8 miles than 3.1. According to the Garmin, that is, but the runners all agreed that the course had to be way off.

You'd think that people would be extra careful in measuring the races, but they're not, I tell you!
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Old 07-06-07, 04:40 PM   #11
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That's not surprising to me.
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Old 07-06-07, 10:28 PM   #12
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I guess I had assumed (somewhat naively) that these people are more careful even when the course is supposedly certified. If I go with my GPS, my 10k time looks much better.

I'm going to try letting it sit out and see if that makes a difference. 'course that will make me look slower while running

Quote:
Originally Posted by mcollet
Its quite possible that the courses that you've been racing at are really off! I own a Garmin, and I believe it is very accurate. There have been a couple of races I've been at this year where the Garmin has measured a shorter course than that it was supposed to be. And I hardly ever race (so there haven't been that many I actually measure).

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