Garmin started measuring short...any ideas?

lastant
lastant Posts: 526
edited June 2014 in Commuting chat
Bit of an odd query, but wondering if anybody else has had their Garmin start measuring short? Spotted this week that my commute is apparently 15% shorter than it was six weeks ago...and we definitely haven't moved!

April 28th, 10.6 miles: http://www.strava.com/activities/142339405

June 4th, 9.2 miles: http://www.strava.com/activities/149509830

Appreciate that GPS recordings will always differ daily due to not following the exact same line but that's a big discrepancy. Any ideas anybody?!
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Comments

  • CiB
    CiB Posts: 6,098
    Are you riding under more tree cover, or past buildings that obstruct satellites, or on v cloudy days when sat signals are weaker? I've seen the same effect for those sort of reasons, where the Garmin fills in the gaps with direct lines between points that overlap omissions. Cumulatively it adds up...

    Alternatively, I well remember Tomorrrows World announcing once that Stoke on Trent was moving closer to London at a rate of a few mm per year (or was it decade?). Either way, that might explain it.
  • lastant
    lastant Posts: 526
    That's the weird thing, looking at the two tracks that have been recorded they are as identical as they could be - it's the same commute along the same roads in central London (the only time it loses satellite reception is under Blackfriars Tunnel).

    It could well be that it's particularly cloudy or something, but by sight they look pretty similar. Certainly not 15% different!
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  • dhope
    dhope Posts: 6,699
    Weird, they look identical. Not added a privacy area since, and Strava is knocking off a .3 mile radius around the start and finish, but the map still looking the same...? No idea tbh.
    You also live right round the corner from me. Prefer NKR/OKR route to Deptford/Surrey Quays?
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  • lastant
    lastant Posts: 526
    May have solved this, turns out my wheel circumference had been reset to 'automatic' so was 18XXmm - have set it to the generic 2096mm once again so will see if normal service is resumed tomorrow. No idea how that happened, but the unit did throw a hissy fit towards the end of the Great Escape audax so could have been then.

    dhope wrote:
    You also live right round the corner from me. Prefer NKR/OKR route to Deptford/Surrey Quays?

    Not sure if 'prefer' is the right word, it's just easier I think - basically head west, turn right, over the river, turn left and head to Westminster. May experiment a bit more now that it's warmer.

    As long as I leave early enough the route's not too bad as is.
    One Man and LEJOG : End-to-End on Two Wheels in Two Weeks (Buy the book; or Kindle it!)
  • Squawk
    Squawk Posts: 132
    I tried noting where the maps noted the 1, 2, 3 mile markers were, and it looks like it's a steady accumulation of inaccuracy as the ride progresses, which really makes things odd. I had thought it might be a big bit wrong in the middle.

    Maybe it's a protest, you're going the wrong way, get off the Old Kent road and up by Rotherhithe roundabout. You can get my scalp up there.

    http://www.strava.com/activities/135036590
  • apreading
    apreading Posts: 4,535
    Wheel circ should be set to automatic - it will be far more accurate as it calibrates from the sattelites over a distance.

    I was about to ask whether you have a speed/cadence sensor and I think the above about wheel circ means that you do. It is probably that the reed in the sensor is shot - this happened to mine and it means that it doesnt record every revolution of the wheel - hence thinking you have not gone as far. they seem weak and loads of people on the internet with same problem. Mine seems to do this when damp but OK when dry. Someone did repair with a new reed and some gaffa tape, but really the only option is a new sensor. If yours is new enough Garmin might replace.
  • lastant
    lastant Posts: 526
    apreading wrote:
    If yours is new enough Garmin might replace.

    Eight weeks new enough?! I'll get onto Garmin and ask about a replacement...
    One Man and LEJOG : End-to-End on Two Wheels in Two Weeks (Buy the book; or Kindle it!)
  • apreading
    apreading Posts: 4,535
    Just make sure that it is not because the magnet is too far from the sensor first - dont want to look stupid!
  • rolf_f
    rolf_f Posts: 16,015
    Significant difference in elevation too......
    Faster than a tent.......
  • EKE_38BPM
    EKE_38BPM Posts: 5,821
    I'm confusedipated. What has magnet location in relation to sensors got to do with GPS?
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  • apreading
    apreading Posts: 4,535
    If you have the speed sensor, the unit uses wheel rotation to measure distance and speed - GPS is just used to record location.
  • lostboysaint
    lostboysaint Posts: 4,250
    Really? My Garmin uses GPS speed. I've also never had the "cloudy", "lots of trees" problem either, despite it being used on the MTB a lot.
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  • apreading
    apreading Posts: 4,535
    lostboysaint wrote:
    Really? My Garmin uses GPS speed. I've also never had the "cloudy", "lots of trees" problem either, despite it being used on the MTB a lot.

    Not is you have a speed sensor attached, it doesnt. It uses GPS to calibrate the sensor and then uses the sensor as it is more accurate.

    Of course, if you dont have a speed sensor then it uses the GPS.

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