thinline
Posts:325
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| 08/04/2014 09:12 AM |
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Okay, what am I not getting? I have gone on a few rides with friends that have trip computers that are more sophisticated than my very basic Cateye with speed, distance etc. They have ones that tell you the total vertical, both ascending and descending, for the course of a ride.
So, when you start and finish a ride in the same place, why don't the ascent and descent totals equal out? Is it just a margin of error in the computation or am I missing some concept here?
It's driving me nutty!  |
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Cosmic Kid
Posts:4209
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| 08/04/2014 10:18 AM |
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Happensd all the time with my Garmins......I just stopped paying attention.
My favorite is when they tell me that I gained or lost elevation during my swims....... |
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| Just say "NO!" to WCP!!!!
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Master50
Posts:340
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| 08/04/2014 11:30 AM |
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| The altimeter function is primarily barometric so minor pressure changes show as altitude changes. Many of these computers also have a GPS altitude feature but the accuracy is likely only 5 meters. so if you are at 6 meters the reading on the GPS could indicate anywhere from 1 to 11 meters and still be within the basic accuracy.
I appreciate that if the gps starts out at 29 meters because you set the altitude and you get home to see 37 meters you still expect the GPS to know this spot is really at 29? Then there are negative readings and we are not in Death Valley. |
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79pmooney
Posts:3189
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| 08/04/2014 01:48 PM |
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The part that bugs me (and shows my age) is that these things add up altitude changes that happen over very small gradients and little bumps of hills that can be rolled over with inertia. I've done organized rides who's literature posts big altitude gains on roads I considered basically flat.
I'm waiting for the day when they get the sensitivity up. They we can post big elevation gains riding chip seal.
Oh, and want to to a really epic climb? One you will never forget? Ride your Garmin equipped trainer in the path of a tornado.
Ben |
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THE SKINNY
Posts:506
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| 08/04/2014 02:06 PM |
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| i wonder if mapmyride is more accurate? i want to say it uses a surface model of the terrain to generate the profile. |
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| How we spend our days is, of course, how we spend our lives. |
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SideBySide
Posts:444
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| 08/04/2014 04:40 PM |
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mapmyride shows my start at 99 feet every time I leave the house. Total gain on my ride to work was 394 and 395 the last two times, so it is pretty consistent. I have it set to autostart and autostop, so there may be small differences due to that.
Edit : feet, not meters. 
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Cosmic Kid
Posts:4209
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| 08/04/2014 05:11 PM |
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Another interesting tidbit re: Garmins and their tracking of elevation gains / losses.
Memorial Day weekend I did 2 loops of the main section of the IM Madison course. Elevation gain was ~4K feet for 80 miles. Did the same course 2 weeks ago and it was roughly half that elevation gain.
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| Just say "NO!" to WCP!!!!
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79pmooney
Posts:3189
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| 08/04/2014 05:21 PM |
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I posted my line about riding a trainer waiting for a tornado thinking of the huge pressure drop as it got near that would register as an epic climb while going nowhere. Then I realized that you just might actually go somewhere!
My advice? Ride the trainer in the basement if you want bragging rights. In the attic and you might not be bragging.
A related true story my grandfather loved to tell was of the man who purchased a barometer in Boston. Brought it back to the store as it was clearly not working. The needle was barely on the scale. The 1938 hurricane struck hours later.
Ben |
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pretender
Posts:46
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| 08/04/2014 06:05 PM |
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| as alluded to above, the precision of any garmin-grade GPS elevation is, at best, about 10 feet. AT ANY GIVEN INSTANT. add all those +/- 10 feet measurements up over even a few laps of the pool and it's gonna be worthless. my work rig is a 25K gps/glonass antenna that i have set so it won't record measurments on a fixed point unless i have at least 20 satellites tracking. i establish control networks by measuring fixed points for 5 minutes each with that antenna, and i still have to go back and manually level the control for elevation. a garmin is probably working off 8 or 10 satellites tops, and using a much more basic processor and algorithm to establish a location. don't expect to get a fine cut with a dull knife. |
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79pmooney
Posts:3189
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| 08/04/2014 06:18 PM |
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pretender, are you saying the Paris-Roubaix climber's jersey is still a few years off?
Ben |
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Orange Crush
Posts:4499
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| 08/10/2014 03:31 PM |
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Posted By SideBy Side on 08/04/2014 04:40 PM
mapmyride shows my start at 99 feet every time I leave the house. Total gain on my ride to work was 394 and 395 the last two times, so it is pretty consistent. I have it set to autostart and autostop, so there may be small differences due to that.
Edit : feet, not meters.
Mapmyride and similar programs are based on digital elevation maps which have limited accuracy. Especially in a forested environment the elevation shown can often be affected by tree tops, leading to a bunch of noise in the elevation profiles that suggest more elevation gained/lost than reality.
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SideBySide
Posts:444
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| 08/11/2014 12:19 PM |
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As long as it registers more gain and loss than reality, then I am OK with it.  I use it because it was free and easy. I am still eying a Garmin 500, but have not pulled the trigger yet.
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gabbard
Posts:27
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| 08/11/2014 12:55 PM |
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When you import GPS data into online programs like Strava or Garmin Connect, there is often an option to do altitude correction. Does this make your data look better? I always assumed the correction algorithms averaged out errors in the GPS data to smooth it out.
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