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Author Topic: Weird mileage differences GPS vs. bike computer  (Read 1554 times)

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Offline Jersh

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Weird mileage differences GPS vs. bike computer
« on: November 11, 2007, 12:54:24 PM »
So I know there's been some previous discussions here concerning the accuracy of GPS data vs. bike computers for keeping track of mileage.  I'm having some weird irregularities with mine and I'm wondering if anyone can offer an explanation... Here are the two scenarios...

1) Yesterday I volunteered over in Clearwater for the Ironman Tri, it was all road miles so I figured the GPS and bike comp would be very close.  GPS recorded 21.49 miles whereas the bike comp reported 25.0.  In this scenario I believe the GPS is accurate and the bike comp is high for some reason.

2)  This morning I did a loop at Morris Bridge.  This was my route... Parked at Trout Creek... Bayshore to Coconut Grove, crossed over to HIF, took Gator Bait to Indian to Washboard to Heartbreak Ridge.  Followed the double track out to the river overlook, stopped for a break.  Took main up to Eubanks Pkwy to the Y, continued on main and took it back to my car at Trout Creek.  Forgot to turn my GPS off, realized it as I was crossing the bridge leaving Trout Creek, so would expect it to be about a mile high.  GPS recorded 24.16, bike comp recorded 16.0.   In this scenario I believe the bike comp is accurate and the GPS is way high for some reason. 

GPS logger is set to record a waypoint every second, have it set this way to avoid the squaring off of round curves when mapping. 

Anyone have any explanation for this?
Josh O'Donnell Photography
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Offline Gregg

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Re: Weird mileage differences GPS vs. bike computer
« Reply #1 on: November 11, 2007, 01:55:58 PM »
Jersh,



Take both things and measure them against a known distance. If you don't know off hand of any, I would suggest the paved loop at Flatwoods. You will know wich one is off. I have a feeling that it is the GPS.

Gregg

Offline FACTORe

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Re: Weird mileage differences GPS vs. bike computer
« Reply #2 on: November 11, 2007, 04:07:03 PM »
GPS will cut off distance and speed on a winding trail.  The way it plots points it squares off all the turns and that is where it loses the time/distance.  I love my GPS for road rides and rides where it isn't slower and technical.  But for MTBing your bike computer will be more accurate.

Offline Jersh

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Re: Weird mileage differences GPS vs. bike computer
« Reply #3 on: November 11, 2007, 04:42:35 PM »
GPS will cut off distance and speed on a winding trail.  The way it plots points it squares off all the turns and that is where it loses the time/distance.  I love my GPS for road rides and rides where it isn't slower and technical.  But for MTBing your bike computer will be more accurate.

Problem is that this morning, my GPS was ~7 miles OVER my bike computer, yesterday it was the exact opposite.
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Offline Gregg

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Re: Weird mileage differences GPS vs. bike computer
« Reply #4 on: November 11, 2007, 04:47:51 PM »
I would bet the bike computer is the one that is accurate.

Offline Jersh

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Re: Weird mileage differences GPS vs. bike computer
« Reply #5 on: November 11, 2007, 05:02:14 PM »
Jonathon, did you have your GPS with you yesterday at the Ironman?  it was a 13.1 mile course, and probably ~3.5 miles to and from each way... I definately don't think we went anywhere near 25 miles like my computer said...
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Offline MtKillanewbieDH

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Re: Weird mileage differences GPS vs. bike computer
« Reply #6 on: November 12, 2007, 08:18:41 AM »
What kind of GPS is it?  I use an Edge 305 (or 205), which has a wheel speed sensor but can also use the GPS to determine distance.  It's my understanding that it uses the distance from the GPS by default if it does not get a signal from the sensor on the wheel. 

You might want to check and see if there is a firmware update for your GPS.  I was just looking around at the Garmin website and found out there is an update for the 305 (I have version 2.90 of the software and the update is to 3.20!).

Here is the link for the Edge 305 update.
http://www8.garmin.com/support/agree.jsp?id=1243
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Offline Jersh

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Re: Weird mileage differences GPS vs. bike computer
« Reply #7 on: November 12, 2007, 11:37:47 AM »
What kind of GPS is it?

It's actually a GPS data logger that is designed for use with my photography...  It's made by a company named GiSTEQ. 

I was in my LBS this morning, and they that likely the GPS and bike comp (which is wireless) are possibly interferring with one another and that would explain the irregularities.
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Offline FACTORe

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Re: Weird mileage differences GPS vs. bike computer
« Reply #8 on: November 12, 2007, 11:57:11 AM »
I forgot to log my miles in for the race and I didn't have my GPS with me...dang it..I think we did 20 miles total

 

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