|Overlay of Garmin track (in red) and race course map created using Google Earth|
The complete track recording can be viewed at Garmin Connect.
The following is my email exchange with Garmin, over the following couple of weeks: Customer Support Form filled out on Garmin website:
Escalation from KANA On Demand Self Service Subject: Erratic track recorded by Forerunner 305
Message: I posted this issue on the Forerunner 305 Forum about 4 weeks ago and did not get any responses: https://forums.garmin.com/showthread.php?t=15785
The track recorded during an xc ski race by my otherwise trusty Forerunner 305 was way off during the second half of the race. This is not the slight 3-4m inaccuracy common in handheld GPS. I can only guess at the cause: cold weather? During the race the temps dropped from -8C (18F) to about -13C (9F). Using Google Earth I created an overlay of the Garmin track from GC and the race course map. Please see this blog entry to view the overlay: http://cfospruns.blogspot.com/2011/01/odd-behavior-of-garmin-fr-305-track.html.
Another odd thing is that the track looks longer than the half-marathon distance covered. GC displays the distance accurately. When I imported to SportsTracks the distance recorded was 26km which seems to be a better match to the displayed track. The complete track recording can be viewed at Garmin Connect http://connect.garmin.com/activity/63570338.
The area where the erratic track is recorded is pretty open there shouldn't be any issues with tall buildings or tree cover (I never had those anyway even in deep woods or big city downtowns). Have you come across this type of problem any possible cause besides the cold?
--- On Sun, 2/13/11, Product.Support@garmin.com
Subject: Re: Erratic track recorded by Forerunner 305
Date: Sunday, February 13, 2011, 3:45 PM
Thank you for contacting Garmin International and be happy to help. We definitely do not want to rule out that the cold weather causing this unusual behavior in your Forerunner 305. However, it might have been two
issues: (1) corrupted data in your satellite data within the GPS or (2) multipath errors.
Corrupted satellite data can occur as the GPS signal comes through the atmosphere. Therefore, if this corruption gets in your device, then it can cause a number of different issues, such as what you experienced within your Forerunner.
If corrupted data was the cause, then you will want to perform an auto locate. This method will erase all satellite data within the Forerunner. I have included these instructions at the bottom of the email.
A multipath error can occur when the GPS signal bounces off an object, like a tree or building, then travels back once it reaches another object. As a result, when the signal reaches the GPS, then the GPS believes you traveled to that area. The only unfortunate part of this is that this error is a limitation of the GPS system, so nothing is able to fix it. However, it should be a rare occurrence that this happen.
With all this information, I would perform the auto locate. Then, please go and do a workout. How does the watch perform?
To perform this auto, please follow these steps:
1. Turn the GPS off
2. Press the Power and the Down button for 15 seconds
3. Set the device outside for a minimum of 30 minutes
Once the 30 minute time frame is completed, then the device should now function correctly. Please test the watch, and if you have any other discrepancies, then please let us know.
With Best Regards,
Product Support Specialist
Original Message Follows: ------------------------
Thanks for your reply.
Since you are not ruling out the cold, I think am going to stick to that unless you provide information on (1) and (2) below.
I am rejecting possibility (1), corrupted satellite data because I found three other Garmin users who uploaded data recorded at around the same time. Their recorded tracks stuck closely to the race course. If I understand your explanation correctly, wouldn't I have noticed discrepancies in their data, too?
Possibility (2), the multi-path error seems far-fetched, too, since when I used the 305 the next day, it worked just fine and it has since January 15, using it four times per week. No problem, this is the only ocurrence since I owned the unit. Also, since this is a rare occurence, would you see this zig-zag pattern over about 10km/40minutes?
Do you recommend I do the auto locate anyway?
I am willing to write this off to a one-time fluke but could you also please confirm whether my understanding of (1) and (2) is correct and tell me whether to do the auto locate?
Thanks for the response and you make good points. I would recommend doing the auto locate on your Forerunner. It does not hurt and it helps to refresh the satellite data. In fact, it is good to do every couple months and when you move the device more than 150 miles with it turned off.
In any case, if the anything else occurs with your Forerunner, then please let us know.
With Best Regards,
Product Support Specialist