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Discussion Starter · #1 ·
I realize the whole GPS/computer accuracy debate has been covered many times over but I feel like I am faster than the GPS unit is telling me. I've been on 5 rides with the new Garmin 500 and I struggle to get the total ride average any higher than 7 or 8 mph. I consider my pace to be really fast and my heart rate says the same as well (never out of Z5). Now I am wondering how accurate are my totals? I did a race last year on the same course I rode tonight and I averaged over 12 mph but the computer says 7 mph. Granted, I was not at race pace tonight but I was not walking. Several times I have looked down when I was just pedaling along and the speed on the GPS unit says 3 or 4 miles per hour and I know I am riding faster than the average person walking :madman: . What are other people seeing with their GPS units?
 

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Two Tired
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You've been reading these forums too much! Everyone else seems to ride faster, jump higher and farther and climb hills much steeper in the forums.

Your average speed will depend a lot on the technical characteristics of the trail and the amount of sustained climbing you do. It is hard to say what your average speed "should" be, but your numbers don't seem that far off if you including climbing and rough trails. Time and distance measurements are pretty close between my GPS and my Cateye (two different ways of measuring).

As an idea, check to see if your Auto Pause is turned on or off. When it is off, your stopped time will be included in your total time measurement, which should reduce your average speed. If you are stopping for friends, opening gates, having a snack, take a picture or whatever, you may be decreasing your average speed. (This is just speculation since the manual is vague and I always have Auto Pause on).

How many walkers are catching and passing you as you ride? If they pass you a lot, it may be accurate! :D
 

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Discussion Starter · #5 ·
My autopause is set to turn off when the bike is stopped. I get the little "beep" when I stop and the little "beep" (auto resume) when I start riding again. I guess I just need to man up and realize that I am not as fast as I thought.

As for walkers passing me, I have not had that much but I was swarmed by a herd of deer that scared the crap out of me last night.
 

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D-Town said:
What are other people seeing with their GPS units?
Here's a video of what I see - watch it in HD to see the GPS kmh and altitude.

The accuracy of the GPS can depend on the terrain. If you are biking in a ravine/canyon,GPS can be fairly inaccurate.
Otherwise my GPS is usually very close to my bikes computer speed and distance traveled.
 

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Complete shot in the dark, but a difference of 12 and 7 almost sounds like your units are set wrong - ya know, kph instead of mph (or vice versa - my head is boycotting math today)?

Probably not it, but worth a mention?
 

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since 4/10/2009
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It could very well be possible that your race pace on a particular track would be 12mph. Are you comparing the same trail or different ones? What may seem fast on one trail could be last place in beginner class on a smooth race course.
 

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A lot of climbimg and descending can though yout GPS off. GPS elevation is not very accurate, and a few miles as the crow flies can be 10 miles with lots of ups and downs. I've also seen my GPS get inaccurate with lots of switchbacks.
 

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Vmax911 said:
A lot of climbimg and descending can though yout GPS off. GPS elevation is not very accurate, and a few miles as the crow flies can be 10 miles with lots of ups and downs. I've also seen my GPS get inaccurate with lots of switchbacks.
a gps, even one with a barometric altimeter, does not use elevation to calculate speed or distance traveled. the gps will only report to you the horizontal distance traveled.

If you upload your track into a program or website afterward that overlays it onto a DEM, it will adjust your distance and speed measurements to include that elevation covered. However, keep in mind that even in that case, most easily available (read: free) DEM data is at a 30m resolution, so it gives areas with a 30m square the same elevation. we all know from experience that terrain can change a lot more quickly, so it leaves out a lot of detail there, too (so when you download to software or upload to a website, and it changes some of your numbers, it's getting closer...just not quite there yet).

yeah, switchbacks can throw off a GPS.

when I map a trail, I prefer to walk it with the GPS set on 1sec recording so I can collect maximum detail for the most accurate maps possible. but at riding speed, small changes like that easily get overlooked.

these inaccuracies, however, aren't going to amount to a 5mph average speed difference on a ride. there's something going on we don't know about.
 

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Not familiar with the 500, but go through its settings. It may not be sampling fast enough for something as slow as a bike. (On my older Garmin Legend it's done from the map page- Setup Map, the Track Log setting. Set it for auto, or small distances.) Also make sure that if it has an on-road and off-road setting that it's set to off-road. My unit records 'moving average' which is different than average. On the roadie, for instance, I probably have an average cruising speed in the 17-19 MPH, but the 'moving average' is lower (14-15 MPH) because I live in a suburban area with a lot of stop signs and stop lights. MA averages out all the time spent slowing and accelerating.
 

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Discussion Starter · #13 ·
More info from today's ride

Thanks for everyone's input. I should state that I know I was not running the course at the same speed as when I raced it. Having said that, I felt like I was moving a lot faster than the GPS was saying

Natehawk, you bring up a very interesting point. I assumed that the distance and speed you see on the GPS is real time and does not require any type of post processing to calculate these values. It would make sense that an algorythm would calculate these values given the unit is capturing X, Y, and Z. I've been using Garmin Training Center and Garmin Connect to enter my ride data. Do these programs calculate the values using the methodology you are talking about?

I rode today with someone who was using a basic computer and he mentioned that the total ride was a little over 19 miles while mine showed 14.4 miles. This of course assumes that he has the right calculation for his wheels but that large of a gap is discouraging to. I have YET to download my data so I'm not sure if the total distance will change.

I will post pack when I run the numbers.
 

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I found that the GPS on my phone (Droid) was about 3 mph off. It was a course with a lot of tight turns and the GPS draws straight lines instead of turns. I was riding on a couse with a confirmed distance. Id expect a true GPS would not be that far off.
 

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I'll admit that I don't know anything about the gps used on mountain biking, but would you be able to bring it in the car with you and see how accurate it is compared to your car? Juts a thought.
 

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trail rat
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D-Town, what software version is in your 500? Garmin frequently has firmware issues in new devices that do get fixed over time. You have a new unit that may have that. Here is what I can find about the updates:

The bold item may be your problem if you have not updated.
Edge 500
Ver. 2.10 - 12/09/2009
Corrected issue transferring heart rate zones and power zones from Garmin Connect and Garmin Training Center.
Corrected issue transferring New Leaf Profiles from Garmin Training Center.
The bike odometer correctly updates during an activity.
Cleanup of TCX conversion issues for courses.
Corrected issues related to courses.
Corrected issue with distance alert performance.
(This version can be downloaded from: Garmin Support Download Section)
http://www.gpsinformation.org/allory/garfeat3.htm
http://www.gpsinformation.org/allory/garfeat3.htm#edge500

Have you downloaded the Garmin Webupdater and run it?
http://www8.garmin.com/support/agree.jsp?id=4715
 

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Training Center and Garmin Connect (and other utilities) all do something to your data after you add it. Problem is, that sort of thing is not documented for public consumption.

Compare your one ride between the raw GPS readout, Training Center, and Garmin Connect. I'm fairly certain all three will be different. They are all going to be estimates. I honestly have no idea which would be closer to actual. I'd bet, however, that sometimes one is closer than the other two and that it can be different between rides.

The firmware update slocaus mentioned sounds like a possible cause of a large difference in expected average speed. To be honest, the average speed you reported was close to mine before I got sick...and I wasn't even remotely in shape back then.
 

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Discussion Starter · #18 ·
slocaus said:
D-Town, what software version is in your 500? Garmin frequently has firmware issues in new devices that do get fixed over time. You have a new unit that may have that. Here is what I can find about the updates:

The bold item may be your problem if you have not updated.

http://www.gpsinformation.org/allory/garfeat3.htm
http://www.gpsinformation.org/allory/garfeat3.htm#edge500

Have you downloaded the Garmin Webupdater and run it?
http://www8.garmin.com/support/agree.jsp?id=4715
I've run the web updater when I first purchased the GPS unit so I am all up to date. It's interesting to see that the odometer was one of their fixes. It might not still be working properly. I'm at the point of contacting Garmin to see if anyone else has the same problem.

The numbers that were reported on the GPS unit at the end of the ride are the same in both GTC and GC (14.4 miles). I'm working on a Mac so I have yet to get into any 3rd party software programs but this is pushing to me to get into Ascent or Rubitracks to see if they can crunch the numbers differently.

I like the idea of taking the GPS in the car to see how it compares. It should provide all the numbers I am looking for as well as providing a good value for vertical distance.

The search continues....
 

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trail rat
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D-Town said:
Thanks for everyone's input. I should state that I know I was not running the course at the same speed as when I raced it. Having said that, I felt like I was moving a lot faster than the GPS was saying

Natehawk, you bring up a very interesting point. I assumed that the distance and speed you see on the GPS is real time and does not require any type of post processing to calculate these values. It would make sense that an algorythm would calculate these values given the unit is capturing X, Y, and Z. I've been using Garmin Training Center and Garmin Connect to enter my ride data. Do these programs calculate the values using the methodology you are talking about?

I rode today with someone who was using a basic computer and he mentioned that the total ride was a little over 19 miles while mine showed 14.4 miles. This of course assumes that he has the right calculation for his wheels but that large of a gap is discouraging to. I have YET to download my data so I'm not sure if the total distance will change.

I will post pack when I run the numbers.
I really think you need to look at the data graphically, and on mapping software, to see what it's telling you, then go from there. I'm still thinking not enough data points resulting in sampling errors. The only way to tell is to overlay what the GPS said you did with a map and see if that's the actual course you took. Another way you could end up with insufficient data is if the GPS was constantly loosing satellite lock and interpolating data.
 
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