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Thread: GPS Accuracy?

  1. #1
    Registered User 37071's Avatar
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    GPS Accuracy?

    I was out riding with friends on Sunday. We got gas and zeroed the odometers on our GPS units. We didn't have 2 of the same model. We rode together for about 200 km and then stopped to talk and checked our GPS odometers. We expected the motorcycle odometers to vary and they did about +/- 7km. The GPS odometers varied about +/- 2km. That is about 1% but I expected better when the predicted accuracy was 3m.
    What is your experience? What factors did we miss?
    Gar

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    Registered User widebmw's Avatar
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    Touring line or racing line ?

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    Alps Adventurer GlobalRider's Avatar
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    Quote Originally Posted by 37071 View Post
    The GPS odometers varied about +/- 2km. That is about 1%
    Comparing one to the other has more to do with precision than accuracy.

    Quote Originally Posted by 37071 View Post
    I expected better when the predicted accuracy was 3m.
    What is your experience? What factors did we miss?
    The GPS updates every second. That 3m is a position accuracy (if you are lucky).

    I'm curious now. I'm going to e-mail Garmin to find out how some calculations are performed.
    Last edited by GlobalRider; 09-24-2008 at 10:52 PM.

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    Benchwrenching PGlaves's Avatar
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    Riding in the big tall trees maybe? Briefly masked signals can cause distance errors. Voni and I rode to and from Alaska this summer. I recorded odometer and GPS readings every day. Almost every day the GPS's recorded the same elapsed distance. I recall only one time the differed by more than 1 mile. I also have two GPS units on my bike - the virtually always read the same elapsed distance.
    Paul Glaves - "Big Bend", Texas U.S.A
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    GPS vs. BMW speedo accuracy

    I am also wondering just how accurate the speedometer component is on my Garmin GPS. I know the BMW speedos are optimistic but I am flying when I'm doing 75mph on my GPS...feels like 90mph. The BMW speedo registers 87 mph when the GPS indicates 75mph. I read all the time about riders on their R75/6 or R90/6 cruising at 90mph and I don't believe it for a moment. I have never had an airhead pass me in my car on the interstate even when I'm driving below 75 and only very rarely by any motorcycle, regardless of make, and never by a Harley. I am wondering if both the speedo and the GPS are off. Also am wondering about the accuracy of the speed riders report cruising at on the interstate as well. What have others noticed.

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    No bugs in winter OHScot's Avatar
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    A little leary myself

    Based on seat of the pants only, using one GPS ever. I think they do not change speed as quickly as the speedo. There is lag time associated with it. If you are running on the freeway steady speed it seems to be accurate. Up and down speed side roads the speedo is moving up and down a lot faster then the GPS.
    "Wow I didn't know BMW made motorcycles, Yeah I think Honda does too."

  7. #7
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by OHScot View Post
    Based on seat of the pants only, using one GPS ever. I think they do not change speed as quickly as the speedo. There is lag time associated with it. If you are running on the freeway steady speed it seems to be accurate. Up and down speed side roads the speedo is moving up and down a lot faster then the GPS.
    Yes - they update once per second.

    If you are doing a linear zero to 60 or 40 to 80 in four or five seconds (example only) then you will see 4 or 5 discreet speeds displayed - one each second as you accelerate and it won't track the speeds in between those 1 second samplings.

    Added: And each sampling will be computed by looking at the distance covered between the point 1 second previous and the current point. Which would display a speed approximately the average speed during that 1 second time period. I can't do the math in my head.
    Last edited by PGlaves; 09-24-2008 at 06:18 PM.
    Paul Glaves - "Big Bend", Texas U.S.A
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  8. 09-24-2008, 05:20 PM

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    Then since the GPS is doing the average speed that would account for the discrepancy between the BMW speedo and the GPS. I had not thought of it that way. The BMW speed is the exact speed at the very moment and the GPS is the average speed of the speed at one second before and the current speed, hence the discrepancy. Thanks for clearing this up. I knew I was going faster than what the GPS was registering. Likewise, I knew I was slowing down faster than the GPS was registering.
    Rb

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    X-Troller hexst's Avatar
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    Did you fill up at the same pump? Did you zero out at the exact same spot? Did you follow one exact track between your group on your 200km ride? 1-2% difference between different riders isn't bad most of us take different lines around curves pass in different areas use the other lanes for different amounts of times.
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  11. #10
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by rbryson View Post
    Then since the GPS is doing the average speed that would account for the discrepancy between the BMW speedo and the GPS. I had not thought of it that way. The BMW speed is the exact speed at the very moment and the GPS is the average speed of the speed at one second before and the current speed, hence the discrepancy. Thanks for clearing this up. I knew I was going faster than what the GPS was registering. Likewise, I knew I was slowing down faster than the GPS was registering.
    Rb
    That is correct - but I will quibble with, "The BMW speed is the exact speed at the very moment." True if your speedo is accurate. None of ours are exact. Typically reading a few (2 to 4) mph high. But yes, the BMW speedo will track linearly while the GPS displays the results of that one per second sampling and distance computing.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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  12. #11
    sumran
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    I ride in Florida. The roads are fairly straight and there are few obstructions for a GPS. I have gotten into the habit of tracking my speed with the GPS, since it is generally more accurate in these riding conditions.

    When I went on vacation in western North Carolina, I discovered that the GPS speed was far less reliable. Using it as an indicator for entry speed into mountain road curves can make them a little too exciting. Luckily, I wasn't pushing the envelope, so the GPS speed error didn't cause big problems.

  13. #12
    RK Ryder
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    On occasion I have looked down at my Street Pilot 2610 to discover that my top speed has been something like 189 mph when in fact it has really been considerably less. This has happened two or three times. When I called Garmin about the inaccurate speed reading, I have been told that when the unit loses satellite reception for a while, the unit takes the reading from the last location and does an analysis of the time taken to get to the present satellite. I have often wondered if a LEO would believe this.
    Paul
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    Pepperfool GSAddict's Avatar
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    After changing the hub on my GS to 2.87:1 my odometer typically reads within 0.3 km of my Zumo 550 over a tankful of gas. Good enough 4 me
    '
    Ufda happens..........

    It's all about the details.

  15. #14
    jcridge
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    Quote Originally Posted by 37071 View Post
    I was out riding with friends on Sunday. We got gas and zeroed the odometers on our GPS units. We didn't have 2 of the same model. We rode together for about 200 km and then stopped to talk and checked our GPS odometers. We expected the motorcycle odometers to vary and they did about +/- 7km. The GPS odometers varied about +/- 2km. That is about 1% but I expected better when the predicted accuracy was 3m.
    What is your experience? What factors did we miss?


    Tires....

    Size, air pressure, manufacture, wear. All 4 of these can affect your recorded mileage by at least 1% or more pending the overall tire diameter variance from factory size and recommended inflation. This is the most common cause in all vehicles on the road today using GPS and trip computers comparing to vehicle speedometer.

  16. #15
    Registered User 37071's Avatar
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    Speed More Accurate?

    I have always heard that BMW Motorcycle speedometers read higher than the actual speed. My Zumo reads lower than the speedometer. Is the speed reading on my Zumo accurate? Has anyone tested their GPS unit with a radar gun?
    Last edited by 37071; 09-26-2008 at 01:21 AM. Reason: spelling
    Gar

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