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645 Altimeter short changing my stats....

livermush
over 4 years ago

After have an old Garmin VivoActive 1 for a long long time I decided to upgrade to the 645M. I love it! However one of the things I was most excited about was finally having a barometric altimeter. I find in general that Strava's recalculation it does on watches that do not have a barometric altimeter (like the VivoActive 1) tends to show less altitude than what you actually did. I'm a stats junkie and I wanted a more accurate reading of my elevation profile from mountain bike rides.

So this is where I was excited to be moving to the 645M...well the excitement did not last long. I find the barometric altimeter to be even less accurate than GPS calculated elevation. The altimeter on the 645M shows me getting even less elevation change over a ride...and I know for a fact after using it a few months that the watch consistently short me on what I actually did. Not by just a little bit either, it's not even close to accurate. A local trail ride that I know has a total ascent of about 1100 feet shows as 750 feet on my 645M. That's pretty disappointing. Anyone else notice this with the 645M? It's really the only issue I have with the watch but for me personally it's a HUGE issue.

  • 0 over 4 years ago

    Have you done the calibration? From what I understand, you shouldn't *need* it, but it can help. I've found mine ok. I wouldn't say it nails it - I've seen 5-10% variance on the same route, but nowhere near the difference you're seeing above...

  • 0 over 4 years ago in reply to stevencbrown

    You just calibrate it by waiting 30 seconds after you turn on GPS before starting your activity right?

  • 0 over 4 years ago in reply to stevencbrown

    And just too be clear I'm not talking about variance between two rides on the same route. The altimeter is super accurate in that aspect....it's just that the altimeter is completely wrong about current elevation, descent, and ascent for a ride. In the example above the true ascent for the ride is 1100 feet and the 645 tells me every time I ride that route that I did 750 feet of ascent.

  • Former Member
    0 Former Member over 4 years ago in reply to livermush

    GPS can be off by + or - 400 feet in elevation.  I don't own a 645, but I'm assuming it will calibrate the same way as my watch.  Go to start your activity. Wait 30 seconds after you get your green GPS fix and then access the altimeter sensor settings and manually correct any offset to your known elevation. 

  • 0 over 4 years ago in reply to livermush

    it can auto calibrate as livermush says, but if you're getting inaccuracies, and you know the elevation where you are starting the run, look here for how to set the elevation manually.

    https://support.garmin.com/en-IN/?faq=xbnpoNIaEDAt8msJht2rM7

  • 0 over 4 years ago

    My 645 worked perfectly last year - matching mapmyride elevation for routes. This year a route that has 1200 feet of climb registers at around 400 total (two tries at the same route yielded the same short-changed results). The issue for me is total elevation change which should not rely on absolute elevation calibration (even though I have calibrated the altimeter in both cases).

    I've got a call into support to see what's up because this is irritating.

    FOLLOW UP: I spoke with a tech who suggested a 1 hour soak in water/dish soap to clear the barometer port. I'll do this and report back. Apparently gunk can slow the response of the barometer to elevation changes during the ride.

  • 0 over 4 years ago in reply to Former Member

    Thanks, I'll give that a try.

  • 0 over 4 years ago in reply to Stackofhay

    Great information...I'll give this a shot too. 

  • 0 over 4 years ago in reply to Stackofhay

    Any luck with soaking the watch i have a 645m and on my last run it recorded -81 elevation.....

  • 0 over 4 years ago

    I've owned a couple of 645 units and the altimeter is very unreliable. It works as expected when new but after 6-12 months it becomes very unreliable. I have tried the wait 30 seconds trick and calibrate by GPS (which doesn't give you an accurate elevation either) but they don't work very well.

    What can really mess the data up is when you stop/start at traffic lights etc. It can suddenly jump up in elevation.

    In my experience, a basic Forerunner without barometric altimeter gives you more reliable and accurate total elevation *most of the time* unless the 645 is brand new device and not from a defective batch. I can also imagine that getting a drop of sweat over the sensor hole will mess up the data. This isn't a problem with GPS only watches.

    Sometimes it works fine, other times the elevation graph looks like a sawtooth or has big, inconsistent jumps. It seems to work better for cycling than running.

    Basically, I think the altimeter in the 645 sucks. It should be redesigned and owners should be offered a discount on an updated 645.