Thread: More Crazy data
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Old April 7th 21, 04:07 AM posted to rec.bicycles.tech
Frank Krygowski[_4_]
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Default More Crazy data

On 4/6/2021 7:24 PM, Mark J. wrote:
On 4/5/2021 2:29 PM, Mark cleary wrote:
I have a Garmin 910xt a triathlon type watch but I am a runner and
cyclist. It supposed to have barometric altimeter in it right? Well
the data is seriously in conflict. I notice all the time when I upload
to Garmin connect then Strava the elevation gain is way less than RIde
With The GPS. I don't use RWGP but it uploads since I have an account.
My Ride with the GPS always shows much more elevation gain. Today I
went 53 miles and on Garmin the gain was 1165 and Ride with the GPS it
was 1630. That is fair amount of off and usually that is close to what
will be. I normally don't track this stuff I live in the flatlands but
the data is suspect for sure. More fake news from something.

Deacon Mark


Elevation gain is inherently a tricky subject, and I suspect even more
so for a rider in a relatively flat area.

Say rider August lives on a route over a mountain pass.Â* The road has a
steady uphill grade, which he rides to the summit and back to his house.
Pretty much any reasonable elevation-gain calculation strategy will work
well for August: Summit Elevation minus house elevation, or elevation
differences every 10 seconds accumulated (and throw out negative
differences that mean he's going downhill) or lots of variations on
those themes.

Contrast rider Bertrand who lives in an area that most cyclists would
call "flat" though in fact the elevation varies by as much as 20 feet at
different places in the county.Â* In fact, the actual elevation is
undulating in a very narrow range.Â* Now the choice of elevation-gain
calculation starts to matter.Â* Suppose you add up the differences every
10 seconds (again throwing out negative differences).Â* If Bertrand
crests a ten-foot-tall hill right in the middle of one of those
ten-second intervals, the net elevation difference over that ten seconds
might end up positive and count, or negative and get thrown out, and in
no case will the height of the "summit" be reflected accurately.Â* It's
reasonable to assume that Bertrand is going to miss a fair bit of
elevation gain if those undulations are frequent.

Now let's grant that the 10-second interval method is a bad method and a
straw man.Â* A one-second interval is probably much more realistic for
modern GPS systems.Â* That will still miss some elevation gain, but
probably not that much.

Oh, but wait - what if the undulations are quite small but high
frequency.Â* Suppose cyclist Conrad was on a route that went up a half
inch in one foot of travel, then down a half inch the next foot,
repeatedly, for 10 miles.Â* Again a weird example, this is gonna feel
like washboard, but how much elevation "gain" does Conrad experience?
Half an inch every two feet for 52,800 feet or 1100 feet of elevation
gain.Â* Should we count that as elevation gain on what's an unnaturally
flat course otherwise? (Highest point minus lowest point is a half inch!)

As for RWGPS, it's not clear to me if RideWithGps is using your GPS
elevation data or just your GPS coordinates and then their own internal
elevation data for those locations.Â* I would actually expect that if
they use their own elevation data, they aren't accumulating elevation
every second, but rather "netting out" elevation gain every ?100 feet?
or so, or perhaps longer.Â* So all sorts of elevation gain could be missed.

As for a GPS device measuring altitude gain, remember that GPS altitude
measurement is prone to significant measurement errors, and even though
the best GPS devices have barometers that "smooth" out the data, GPS
data is going to have tiny random, jiggly errors.Â* Those errors in
once-per-second measurements (a guess at frequency) can add up just like
Conrad's weird elevation "gain."

All that said, I would not expect RWGPS figures to be *higher* than the
Garmin figures, but maybe I'm just mis-estimating the magnitude of each
issue, for which my illustrative examples are intentionally extreme.
Still, I suspect that these are some of the issues that are involved.

Finally, there's an inherent problem in the original "question" of
elevation gain.Â* Route profiles are probably fractal if we look closely
enough.Â* Measure (accurately) many times per second or per foot, and we
will accumulate lots of ?superfluous? elevation gain like Conrad.Â* It
will get worse as frequency of measurement goes up. [1] In this way,
Conrad's example illustrates my central point - we have to decide /what
counts/ as elevation gain.Â* Our rough estimates can agree "well enough"
for generous values of "enough," but they are never going to agree
exactly unless we have a fixed measurement method.

[1] Google "How long is the coastline of Britain" to find articles about
fractals and Mandelbrot, etc.

Mark Janeba


Nicely done. About halfway through that, I decide to bring up fractals
and coastlines. You beat me to it.

--
- Frank Krygowski
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