Not Happy With AW's Measurement of Distances

Discussion in 'Apple Watch Fitness and Health' started by ArubaKid, May 1, 2015.

  1. ArubaKid

    ArubaKid Member

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    Today I used the Workout app to gather data on my 2.5 mile outdoor exercise walk. As I am walking I can view the current time of the walk, distance, average heartbeat, etc, including a "minutes per mile" calculation. I usually walk at a rate of a hair over 15 minutes per mile. As I look at this number as I walk, sometimes it reads 15 min/mi, sometimes over 17min/mi. And it changes values as I walk pretty much at the same speed. This isn't right, and it got me to thinking.

    So I took the AW (with my iPhone 6 on my belt) to my driveway when I got home and did some tests. I measured a 76 foot distance, and took several readings as I walked this distance, using Outside Walk on the Workout app. I must have walked this 76 foot track about 20 times and took a reading of distance at the end of each trial. Low and behold readings varied all over the lot from 22 feet to 44 feet to 78 feet to 122 feet! Huh? (Note that I wasn't trying to record my speed or how long it took me to walk the 76 feet. I was just interested in how long the AW thought the 76 feet were. Answer: anywhere from 22 feet to 122 feet.

    And the way the readings were displayed was weird: it would stay at zero of a long time than jump to 12 feet then stay at 12 and jump to 50, and then jump to 90... Yes, the iPhone was connected to the AW the whole time.

    My conclusion: the AW/iPhone combination can't measure distance worth a darn.

    The AW was touted as a great device for fitness, and it can't measure distance for outside walking and jogging and running? Bah!

    Of course 76 feet is a pretty short distance compared to the much longer distances that people will walk/jog/run when they are working out. But I did not like the way that the distance stayed at one number and then jumped to a much bigger number. If I had hit pause or stop anywhere while the distance wasn't changing I would get wrong calculations. I expected the distance to change regularly every few seconds, and readings were communicated from the iPhone' GPS to the AW. Go figure. I wonder how often GPS readings are taken by the iPhone. If it is every 5 seconds people running on a circular track are going to get bogus numbers if the iPhone or AW computes distances by subtracting GPS readings.

    Any thoughts?
     
  2. Jeffrey

    Jeffrey Member

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    I would think you would get better readings. Perhaps it's a hardware issue and the watch needs to be replaced.
     
  3. twerppoet

    twerppoet Well-Known Member

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    My understanding is that as long as you carry the iPhone, the distance readings should start to get more accurate. The watch's activity app uses the accelerometer to count your steps and estimate the distance. By comparing that to the GPS on the phone, little by little the app becomes more accurate in estimating your average stride length.

    Anyway, there have been one or two blog posts on this issue, where the blogger said the app started getting more accurate after a few activities; until they were able to leave the iPhone behind and still get a reasonably accurate distance measurement.

    I think the watch pretty much has to do it this way. If it depended on the GPS for actual distance measurements then it would be useless as a stand alone fitness tracker; one of the few things it is supposed to be able to do with out the iPhone near by.

    As for your test, 76 feet isn't a good distance to be walking to help the GPS calibrate the watch. GPS accuracy is only 15 feet (at best), which makes for a huge margin of error. A better test would be to walk 1 mile or 2 in big obvious sections. Once you've done that, you could try the 76 feet, counted steps, distance test again and see how it fairs.
     
  4. Jeffrey

    Jeffrey Member

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    Calibration my be necessary. Here"s how....

    upload_2015-5-2_14-59-8.png
     
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  5. col.bris

    col.bris Administrator Staff Member

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    Great information. I understand the op's concern and the effort he made testing however at the same time we need to look at the issue and make sure that it is not just a calibration issue.
     
  6. ArubaKid

    ArubaKid Member

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    Very helpful, Jeffrey. Thank you very much. I will keep using the workout walk outdoors app and see how it improves. (BTW, my settings were all set perfectly. All I have to do is keep walking.)
     
  7. twerppoet

    twerppoet Well-Known Member

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    Good Info, Jeffrey.

    Let me expand a bit on why I don't think the 76 foot test/calibration is usable.

    As I already said, the GPS can be off by 15 feet, even under the best conditions. It could be in the same direction on both ends, which would be a wash. It could be up to 15 feet short on each end, which would mean the iPhone thinks you moved 46 feet. It could be long on both ends which means the iPhone things you moved 106 feet. Or it could be anything in-between.

    Lets look at the worst case. The difference between 46 feet and 106 feet is 60 feet. Lets say you've got a longish stride, and took about 25 steps (about three feet per step). If the iPad depends purely on the GPS data and step count (which it does not), then it would think your stride is anywhere between 1.8 feet and 4.25 feet. This is a potential error of over 2 feet per step.

    Now lets up those numbers, so that the endpoints (where you change or reverse directions) are further apart. Say 3000 feet (a little over half a mile). So, that would be 1000 steps using my earlier long stride stick. But only the original 60 feet difference in maximum and minimum GPS measures.

    The shortest distance computes as (3000 - 60) / 1000 = 2.94 feet per stride
    The longest is (3000 + 60) = 3.06 feet per stride

    Only a .12 foot difference.

    The larger the numbers, the smaller the potential for error should shrink. Also, errors should be weeded out by multiple walks. Given enough walks the GPS errors should average out. No idea how many walks that would take.

    Now I don't think this is exactly how Apple is doing things. There's bound to be lots of corrections based on the accelerometers, GPS accuracy level, and other things I can't think of, but it's bound to be a contributing factor when the GPS is being used to calibrate.

    So, when calibrating, take some nice, pleasant, long walks. Once you've done that, leave the phone inside and try your test again. Hopefully you'll see some serious improvements.


    You probably understood all this the first time, but I really really felt like explaining. Sorry. :)
     
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  8. watchfuleye

    watchfuleye New Member

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    Some great info from twerppoet in this thread. I knew that it was off the first time I took my Sport for a run so didn't bother using the distance tracker after that. Maybe I'll give it another go :)
     
  9. twerppoet

    twerppoet Well-Known Member

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    Let us know how it does.

    No estimate is ever going to be as good as an active GPS, but it's nice to have an idea of how close the estimate is. And I'm too lazy (and addicted to having my iPhone with me at all times) to experiment
     
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