Re: Difference between GPS and measured distace.

On 7/28/2024 5:19 PM, Roger Merriman wrote:

AMuzi <am@yellowjersey.org> wrote:

On 7/28/2024 2:04 PM, Tom Kunich wrote:

On Sun Jul 28 17:57:01 2024 Roger Merriman  wrote:

Tom Kunich <cyclintom@yahoo.com> wrote:

T have been doing a route past the airport and back that has pretty large
variations in distance on the Garmin but mostly 25 miles. Yesterday I
installed a wheel sensor on the bike and added it to the sensor group.
The 25 mile route came in at 27 miles. Now upon occasion it showed that
distance on straight GPS but it appears that civilian GPS is pretty
easy to interfere with.
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You?ve done something wrong to be getting that degree of inaccuracy. If
it?s GPS drift is possible to map that.
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I use GPS only, but even so very minimal deviations certainly not into
miles or kilometres!
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Roger Merriman
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This only occurs around the airport and not up in the hills. So I think
that it is due to radio interference.

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or perhaps:
https://spectrum.ieee.org/faa-files-reveal-a-surprising-threat-to-airline-safety-the-us-militarys-gps-tests
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https://www.gps.gov/spectrum/
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https://arstechnica.com/information-technology/2022/10/cause-is-unknown-for-mysterious-gps-outage-that-rerouted-texas-air-traffic/
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Nice phrase in that one, "experiencing GPS anomalies".  Just
like you!
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Seems unlikely considering for Tom’s cause more likely frankly set up.
 I ride past or through Heathrow on the commute! It’s within 0.06 miles let
alone the military stuff just to the north. Which is generally same sort of
tolerance one gets riding with others ie the difference, between folks
recorded distance.
 And Tom’s device almost certainly would be multi band ie uses either the
Russian or European alternative to get more points and thus more accurate
though I believe most devices are fairly good and frankly for roads it’s
kinda a non issue, used to get some drift in the woods and so on but it’s
at least from a user experience spot on.
 Note that for MTB trails may run parallel and within a few meters but
modern devices don’t wander and can even use foresight to tell you which
trail your pointed at, which is often not clear ie multiple trails all
heading off from a clearing in the woods!

I'm wondering why this even matters. It sounds like Tom's distance measurements are within 4% of each other. Why does he need it to be closer?
My first cyclometer was a little metal "tick-tick-tick" one driven by a steel peg clamped to a spoke. Its calibration was whatever the manufacturer decided was correct for a nominal 27" wheel.
I think the manufacturer was Lucas, like this one:
https://rustybikebell.wordpress.com/2012/01/03/the-lucas-king-of-the-road-cyclometer/
It stopped working within just a few months. I thought that was surprising for a totally sealed mechanical device. But that was before I heard about Lucas electric devices.
"James Lucas, the inventor of the intermittent windshield wiper. Also the intermittent headlight, intermittent ignition system, intermittent brake lights..."
--
- Frank Krygowski