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Garmin 305 off by1.5 mi?

Old 01-21-10, 08:02 PM
  #1  
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Exclamation Garmin 305 off by1.5 mi?

So I went out for a nice 40 mi ride, 3rd ride since I got it for Christmas/Birthday. Today I noticed that when I was looking at the route versus the speed/cadence, it did not seem to line up. There was one point where we got to a nice long lonely staight away and I did a nice sprint up to max heart rate ,and then a cool down till the pack caught up.

About 10 miles later we went out 5 mi in a nice paceline, stopped for water, and then returned and stopped again at the park for refills and a potty break.

In all 3 cases the speed "peak" in the case of the sprint or the cadence and speed "valley" in the case of the rest stops, all seem to be off by about 1.5 mi compared to the map. I understand the the GPS is accurate to maybe 100 meters, but 1.5 mi?

Has anyone else had anything similar?
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Old 01-21-10, 10:37 PM
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Reride the same route and see what the results are. Typically the sats atomic clocks experience noise and clock drift errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state.
These problems tend to be very small, but may add up to tens of feet of inaccuracy which is in constant flux. Thus reriding the route possibly could give you a different reading.
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Old 01-21-10, 11:39 PM
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You are comparing your "track" line to a map image and since you see where you rode "in the image" and your track line is not exactly where you rode "in the image", you assume something is wrong with your GPS. The problem is ... sometimes the map images are just not that "in alignment" with GPS coordinates.

Some maps are very "in sync" with GPS coordinates and some ... not so much. Do like 'froze' said and reride the route. If your tracks repeatedly overly each other on the map, then your GPS coordinates are probably OK and its just the map "misalignment" thing. If you ride the route again and the track is substantially different than the first, then it might be a GPS accuracy thing.

I always assume the track is more accurate than where it lands on the map image.
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Old 01-22-10, 11:07 AM
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Originally Posted by bobthib View Post
So I went out for a nice 40 mi ride, 3rd ride since I got it for Christmas/Birthday. Today I noticed that when I was looking at the route versus the speed/cadence, it did not seem to line up. There was one point where we got to a nice long lonely staight away and I did a nice sprint up to max heart rate ,and then a cool down till the pack caught up.

About 10 miles later we went out 5 mi in a nice paceline, stopped for water, and then returned and stopped again at the park for refills and a potty break.

In all 3 cases the speed "peak" in the case of the sprint or the cadence and speed "valley" in the case of the rest stops, all seem to be off by about 1.5 mi compared to the map. I understand the the GPS is accurate to maybe 100 meters, but 1.5 mi?

Has anyone else had anything similar?
This is confusing. It looks like you are thinking about speed (miles per hour) but talking about distance (1.5 miles).

GPS units measure position. They determine speed indirectly by computing the difference in position divided by the time interval between the two positions.

The larger the distance between the two positions, the more accurate the speed determination will be.

Given how slow bicycles travel, the +-300 feet accuracy in position can cause large "fake" differences in speed. The GPS unit can reduce reporting wacky data due to inaccuracies in position by averaging across multiple positions.

Last edited by njkayaker; 01-22-10 at 11:13 AM.
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Old 01-22-10, 02:40 PM
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Say what you will, but I'm now convinced the problem lies with the Garmin mapping software. I have evidence to the contrary. Yesterday I was unable to upload the ride to my favorite mapping site, http://www.ridewithgpd.com but I reported the problem and they diagnosed and fixed the problem yesterday afternoon. Today I uploaded the ride and the speed peak and the two rest stops are spot on as compared to the route map. So the problem has to do with how the Garmin sites are interpreting the GPS data!

I know that RWGPS uses Google maps. Don't know what Garmin uses. Interesting! Garmin is a GPS company and can't get it right!
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Old 01-22-10, 02:53 PM
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Originally Posted by bobthib View Post
... but I reported the problem and they diagnosed and fixed the problem ...
I'm really curious. What was the diagnosis and what was the fix??
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Old 01-22-10, 09:44 PM
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Originally Posted by bobthib View Post
I know that RWGPS uses Google maps. Don't know what Garmin uses. Interesting! Garmin is a GPS company and can't get it right!
NONE of the current GPS companies on the market can get it right...yet.

Interesting thing in Europe is that some drivers follow their GPS's blindly, so much so that cops have responded to drivers who drove into fields, and when asked why they drove into the field the drivers responded that the GPS showed there was a road where they were at!!! And you find the same thing here, GPS's showing roads where there is none, or the address is off by a mile or more etc.

All GPS's will give you a different route if you enter a long complicated designation, not that neither is wrong they just will route you differently. For example I went on business trip and for fun one of the guys brought his Tom Tom and another brought his Megellan I brought along my Garmin. To get from Fort Wayne IN to Columbus OH, the Garmin wanted me to take 27 to 33 to 270 in Columbus, the Tom Tom said to take 30 to 23 to 270 in Columbus and the Megellan said to take the 30 to 68 to the 31 to the 33 to 270 in Columbus. These GPS's are not perfect. But these routing differences can also be found on any online mapping program as well. Which way was the fastest? We couldn't tell so we took he Garmin route since it seemed more direct.
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Old 01-23-10, 05:23 AM
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Originally Posted by head_wind View Post
I'm really curious. What was the diagnosis and what was the fix??
Head, this reference is to not being able to use Garmin Sync on RWGPS, not the mileage problem with Garmin Connect or TC. The Sync Problem at RWGPS had to do with a site update that broke the Sync. They fixed it immediately when reported.
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Old 01-23-10, 05:28 AM
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Originally Posted by froze View Post
NONE of the current GPS companies on the market can get it right...yet.

Interesting thing in Europe is that some drivers follow their GPS's blindly, so much so that cops have responded to drivers who drove into fields, and when asked why they drove into the field the drivers responded that the GPS showed there was a road where they were at!!! And you find the same thing here, GPS's showing roads where there is none, or the address is off by a mile or more etc.

All GPS's will give you a different route if you enter a long complicated designation, not that neither is wrong they just will route you differently. For example I went on business trip and for fun one of the guys brought his Tom Tom and another brought his Megellan I brought along my Garmin. To get from Fort Wayne IN to Columbus OH, the Garmin wanted me to take 27 to 33 to 270 in Columbus, the Tom Tom said to take 30 to 23 to 270 in Columbus and the Megellan said to take the 30 to 68 to the 31 to the 33 to 270 in Columbus. These GPS's are not perfect. But these routing differences can also be found on any online mapping program as well. Which way was the fastest? We couldn't tell so we took he Garmin route since it seemed more direct.
I agree about mis-labeled streets and weird routing. We have a few Tom Toms, and I use Google Maps and Mapquest when I'm in the house and want to get an idea on how to go to a new destination.

But this is something different. This has to do with the way that the TC and Garmin Connect are processing the data file direct from the Edge 305. The data are correct. The sync as between the histograms are the route traced on the maps is wrong.

That same data uploaded to the RWGPS does not loose the sync.
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Old 01-23-10, 09:55 AM
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FYI - I uploaded todays 46 mi ride into TC, Garmin Connect, and RWGPS. Interestingly, RWGPS and Garmin Connect both lined up so that the speed drops occurred at the points where the rest stops occurred. TC was still off by about 1.5 mi.
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Old 01-23-10, 01:01 PM
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PLEASE NOTE. Someone in the Road forum asked me if the answer to my "Headline" question is NO.

That is correct. The Garmin 305 is not the problem. The GPS data are correct. The problem lies in the software. Now it is only in TC. Garmin has corrected the Connect website.

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Old 01-23-10, 08:21 PM
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Originally Posted by bobthib View Post
I know that RWGPS uses Google maps. Don't know what Garmin uses. Interesting! Garmin is a GPS company and can't get it right!
They get the GPS part right. You obviously don't realize that none of them make their own maps. And TC is known inaccurate with just the PC basemaps. If you don't want to buy maps download the Ibycus maps (freeware)
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Old 01-24-10, 09:22 AM
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Originally Posted by CCrew View Post
They get the GPS part right. You obviously don't realize that none of them make their own maps. And TC is known inaccurate with just the PC basemaps. If you don't want to buy maps download the Ibycus maps (freeware)
I do know that they license their map software, but I don't know from who. Indeed the TC maps are lousy and the even warn you that it's not very good, and no update is available from Garmin. And from what I see on the Ibycus site, they are Canada only. Not much help in FLA.

Let me say, however, the problem is not accuracy of the map. The route falls directly on the roads traveled. The problem has to do with synchronization of the histogram data to the position on the route track. The timing is off by about 1.5 mi or 6 min at pace.

THe problem was also on the Garmin Connect site, but that got corrected.
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Old 01-24-10, 10:46 AM
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Originally Posted by bobthib View Post
. And from what I see on the Ibycus site, they are Canada only. Not much help in FLA.
Garmin's maps are licensed from Navtec

You're jumping to conclusions pretty quickly it seems. Ibycus makes a full USA mapset. http://www.gpsfiledepot.com/maps/view/19

Interesting that I've run a 305 and I have over a years worth of data with no issues

Last edited by CCrew; 01-24-10 at 10:53 AM.
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Old 01-24-10, 11:50 AM
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Originally Posted by CCrew View Post
Garmin's maps are licensed from Navtec

You're jumping to conclusions pretty quickly it seems. Ibycus makes a full USA mapset. http://www.gpsfiledepot.com/maps/view/19

Interesting that I've run a 305 and I have over a years worth of data with no issues
I googled Ibycus and the site only had canada. Perhaps they have US, but then I'm really not interested. Note also the problem is not the 305. It has to do with TC. And it's not a big deal. I'm just puzzled as to why it happends.
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