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  #21  
Old March 6th 19, 01:41 AM posted to rec.bicycles.tech
John B. Slocomb
external usenet poster
 
Posts: 805
Default GPS (was: Wheel weight)

On Wed, 6 Mar 2019 00:23:00 +0000 (UTC), Claus A▀mann
wrote:

John B. Slocomb wrote:

I remember back when we lived on the boat the GPS would sometimes
measure the altitude at 10 feet which was about twice the height above


That's why even most GPS based cycle computers use barometric pressure
for altitude -- GPS is pretty bad for that.


"Note: please read the netiquette before posting. I will almost never
reply to top-postings which include a full copy of the previous
article(s) at the end because it's annoying, shows that the poster
is too lazy to trim his article, and it's wasting the time of all
readers."

If you don't care to answer then just don't bother. It is not that you
would be missed.

--
Cheers,
John B.


Ads
  #22  
Old March 6th 19, 01:47 AM posted to rec.bicycles.tech
John B. Slocomb
external usenet poster
 
Posts: 805
Default Wheel weight

On Tue, 5 Mar 2019 16:44:03 -0800, "Mark J."
wrote:

On 3/5/2019 3:20 PM, John B. Slocomb wrote:
On Wed, 6 Mar 2019 09:41:29 +1100, James
wrote:

On 6/3/19 2:48 am, wrote:
On Tuesday, March 5, 2019 at 7:45:36 AM UTC-8, wrote:


The carbon clinchers: Front; 1.13 Kg Rear with 11-29 cassette; 1.58 Kg.

That is with tires and tubes. And the speedo magnet.


If you changed to a Garmin or other GPS based speedometer, you could
save valuable grams from the front wheel because there's no need for a
magnet.


I've always been a little skeptical about GPS calculated measurements.
I remember back when we lived on the boat the GPS would sometimes
measure the altitude at 10 feet which was about twice the height above
sea level that the receiving antenna was mounted at.


As you should be; GPS has a notoriously large margin of error for
measuring altitude.

I think it's the trigonometry of the computation; the GPS (as I
understand it) measures distance to a collection of satellites whose
positions are well known, then computes location from triangulating the
results. I'm guessing that since most of the satellites are usually not
directly overhead, but rather the line of sight to the satellite is
usually be much closer to tangential to the earth, then very small
errors in the distance-to-satellite computation turn into much larger
errors in the altitude computation.

I think this is why higher-end bike GPS's have a pressure-based
altimeter as well, to correct the fluctuations in the GPS-computed
altitude. I know my Garmin Edge's regularly solicit known altitude
input at the start of a course.

Mark J.


I'm not sure about how accurate GPS really is but back in the day, the
seismic folks had a large "Black Box" that they used to locate their
seismic lines on the chart that they said was accurate to within feet.

--
Cheers,
John B.


  #23  
Old March 6th 19, 03:28 AM posted to rec.bicycles.tech
James[_8_]
external usenet poster
 
Posts: 5,932
Default Wheel weight

On 6/3/19 12:47 pm, John B. Slocomb wrote:
On Tue, 5 Mar 2019 16:44:03 -0800, "Mark J."
wrote:


I think it's the trigonometry of the computation; the GPS (as I
understand it) measures distance to a collection of satellites whose
positions are well known, then computes location from triangulating the
results. I'm guessing that since most of the satellites are usually not
directly overhead, but rather the line of sight to the satellite is
usually be much closer to tangential to the earth, then very small
errors in the distance-to-satellite computation turn into much larger
errors in the altitude computation.

I think this is why higher-end bike GPS's have a pressure-based
altimeter as well, to correct the fluctuations in the GPS-computed
altitude. I know my Garmin Edge's regularly solicit known altitude
input at the start of a course.


I'm not sure about how accurate GPS really is but back in the day, the
seismic folks had a large "Black Box" that they used to locate their
seismic lines on the chart that they said was accurate to within feet.


Mark is pretty close to the correct reason. Most GPS receivers
intentionally track satellites that are close to the horizon as opposed
to direct overhead. This is so that the X-Y part of the position
information is most accurate, at the expense of less accurate Z position.


The overall accuracy depends greatly on the GPS receiver quality. The
cheap receivers (say $50 ea) may be within a few meters, while expensive
receivers ($500) are 10 times better. If you pay more ($10,000) and
incorporate corrections for atmospheric conditions and such, accuracy
can be better still.

The difference between cheap and expensive is largely down to the
stability of the oscillator used to time signals. The antennas can also
be an expensive part and play a big role in accuracy and reflected
signal rejection.

But... Even cheap GPS receivers are relatively stable over a short
time. They usually produce a position, speed and heading once per
second. The previous position, speed and heading are combined with new
measurements in a special filter, that usually results in better
accuracy than if the measurements were used alone.

The only times I've noticed real problems is when you cycle relatively
fast around tight corners. The GPS position effectively cuts a little
off the corner each time, modelling it as a series of straight lines.
Hence your road speed appears to be slower than it really is and you
appear to accelerate again when the road straightens out.

More expensive GPS receivers can produce calculated position results
more frequently than 1 per second. More powerful processor. More power
consumption. Unlikely to be in a battery operated consumer grade bike
computer.

But I find that they are accurate enough not to miss the magnet and reed
switch.

--
JS
  #24  
Old March 6th 19, 03:53 AM posted to rec.bicycles.tech
Frank Krygowski[_4_]
external usenet poster
 
Posts: 7,283
Default Wheel weight

On 3/5/2019 10:28 PM, James wrote:
On 6/3/19 12:47 pm, John B. Slocomb wrote:
On Tue, 5 Mar 2019 16:44:03 -0800, "Mark J."
wrote:


I think it's the trigonometry of the computation; the GPS (as I
understand it) measures distance to a collection of satellites whose
positions are well known, then computes location from triangulating the
results.┬* I'm guessing that since most of the satellites are usually not
directly overhead, but rather the line of sight to the satellite is
usually be much closer to tangential to the earth, then very small
errors in the distance-to-satellite computation turn into much larger
errors in the altitude computation.

I think this is why higher-end bike GPS's have a pressure-based
altimeter as well, to correct the fluctuations in the GPS-computed
altitude.┬* I know my Garmin Edge's regularly solicit known altitude
input at the start of a course.


I'm not sure about how accurate GPS really is but back in the day, the
seismic folks had a large "Black Box" that they used to locate their
seismic lines on the chart that they said was accurate to within feet.


Mark is pretty close to the correct reason. Most GPS receivers
intentionally track satellites that are close to the horizon as opposed
to direct overhead.┬* This is so that the X-Y part of the position
information is most accurate, at the expense of less accurate Z position.


The overall accuracy depends greatly on the GPS receiver quality.┬* The
cheap receivers (say $50 ea) may be within a few meters, while expensive
receivers ($500) are 10 times better.┬* If you pay more ($10,000) and
incorporate corrections for atmospheric conditions and such, accuracy
can be better still.

The difference between cheap and expensive is largely down to the
stability of the oscillator used to time signals.┬* The antennas can also
be an expensive part and play a big role in accuracy and reflected
signal rejection.

But...┬* Even cheap GPS receivers are relatively stable over a short
time.┬* They usually produce a position, speed and heading once per
second.┬* The previous position, speed and heading are combined with new
measurements in a special filter, that usually results in better
accuracy than if the measurements were used alone.

The only times I've noticed real problems is when you cycle relatively
fast around tight corners.┬* The GPS position effectively cuts a little
off the corner each time, modelling it as a series of straight lines.
Hence your road speed appears to be slower than it really is and you
appear to accelerate again when the road straightens out.

More expensive GPS receivers can produce calculated position results
more frequently than 1 per second.┬* More powerful processor.┬* More power
consumption.┬* Unlikely to be in a battery operated consumer grade bike
computer.

But I find that they are accurate enough not to miss the magnet and reed
switch.


One interesting effect I've noticed is that our car's GPS shows our
progress while we're in a tunnel.

I assume it's just extrapolating from our speed when we entered. I
haven't had the chance (nor desire) to slow down radically while in a
tunnel to test it.


--
- Frank Krygowski
  #25  
Old March 6th 19, 03:55 AM posted to rec.bicycles.tech
JBeattie
external usenet poster
 
Posts: 3,898
Default Wheel weight

On Tuesday, March 5, 2019 at 7:28:17 PM UTC-8, James wrote:
On 6/3/19 12:47 pm, John B. Slocomb wrote:
On Tue, 5 Mar 2019 16:44:03 -0800, "Mark J."
wrote:


I think it's the trigonometry of the computation; the GPS (as I
understand it) measures distance to a collection of satellites whose
positions are well known, then computes location from triangulating the
results. I'm guessing that since most of the satellites are usually not
directly overhead, but rather the line of sight to the satellite is
usually be much closer to tangential to the earth, then very small
errors in the distance-to-satellite computation turn into much larger
errors in the altitude computation.

I think this is why higher-end bike GPS's have a pressure-based
altimeter as well, to correct the fluctuations in the GPS-computed
altitude. I know my Garmin Edge's regularly solicit known altitude
input at the start of a course.


I'm not sure about how accurate GPS really is but back in the day, the
seismic folks had a large "Black Box" that they used to locate their
seismic lines on the chart that they said was accurate to within feet.


Mark is pretty close to the correct reason. Most GPS receivers
intentionally track satellites that are close to the horizon as opposed
to direct overhead. This is so that the X-Y part of the position
information is most accurate, at the expense of less accurate Z position.


The overall accuracy depends greatly on the GPS receiver quality. The
cheap receivers (say $50 ea) may be within a few meters, while expensive
receivers ($500) are 10 times better. If you pay more ($10,000) and
incorporate corrections for atmospheric conditions and such, accuracy
can be better still.

The difference between cheap and expensive is largely down to the
stability of the oscillator used to time signals. The antennas can also
be an expensive part and play a big role in accuracy and reflected
signal rejection.

But... Even cheap GPS receivers are relatively stable over a short
time. They usually produce a position, speed and heading once per
second. The previous position, speed and heading are combined with new
measurements in a special filter, that usually results in better
accuracy than if the measurements were used alone.

The only times I've noticed real problems is when you cycle relatively
fast around tight corners. The GPS position effectively cuts a little
off the corner each time, modelling it as a series of straight lines.
Hence your road speed appears to be slower than it really is and you
appear to accelerate again when the road straightens out.

More expensive GPS receivers can produce calculated position results
more frequently than 1 per second. More powerful processor. More power
consumption. Unlikely to be in a battery operated consumer grade bike
computer.

But I find that they are accurate enough not to miss the magnet and reed
switch.


I have no instrumentation. I like surprises at the end of the ride when I ask my fully instrumented riding buddies how far we went and how much we climbed -- then I round up. No data to prove me wrong. I adopt my son's power data when we're riding together since we're both about the same weight, although he is all muscle and I'm muscle and other things. He gave me a Stages GPS Garmin-ish thing from work, but I haven't put it on my bike. It sends me an e-mail every week reminding me that I haven't ridden any miles. That's super helpful.

-- Jay Beattie.




  #26  
Old March 6th 19, 03:59 AM posted to rec.bicycles.tech
Frank Krygowski[_4_]
external usenet poster
 
Posts: 7,283
Default GPS

On 3/5/2019 8:41 PM, John B. Slocomb wrote:
On Wed, 6 Mar 2019 00:23:00 +0000 (UTC), Claus A├čmann
wrote:

John B. Slocomb wrote:

I remember back when we lived on the boat the GPS would sometimes
measure the altitude at 10 feet which was about twice the height above


That's why even most GPS based cycle computers use barometric pressure
for altitude -- GPS is pretty bad for that.


"Note: please read the netiquette before posting. I will almost never
reply to top-postings which include a full copy of the previous
article(s) at the end because it's annoying, shows that the poster
is too lazy to trim his article, and it's wasting the time of all
readers."

If you don't care to answer then just don't bother. It is not that you
would be missed.


It is good advice, though. Some of these threads have extensive
repetitions that are over 10 levels deep.


--
- Frank Krygowski
  #27  
Old March 6th 19, 04:11 AM posted to rec.bicycles.tech
John B. Slocomb
external usenet poster
 
Posts: 805
Default GPS

On Tue, 5 Mar 2019 22:59:03 -0500, Frank Krygowski
wrote:

On 3/5/2019 8:41 PM, John B. Slocomb wrote:
On Wed, 6 Mar 2019 00:23:00 +0000 (UTC), Claus A▀mann
wrote:

John B. Slocomb wrote:

I remember back when we lived on the boat the GPS would sometimes
measure the altitude at 10 feet which was about twice the height above

That's why even most GPS based cycle computers use barometric pressure
for altitude -- GPS is pretty bad for that.


"Note: please read the netiquette before posting. I will almost never
reply to top-postings which include a full copy of the previous
article(s) at the end because it's annoying, shows that the poster
is too lazy to trim his article, and it's wasting the time of all
readers."

If you don't care to answer then just don't bother. It is not that you
would be missed.


It is good advice, though. Some of these threads have extensive
repetitions that are over 10 levels deep.


True, but it is nice sometimes, to go back to earlier posts to
demonstrate that the poster is changing the subject rather than admit
that he really didn't know what he was talking about.

--
Cheers,
John B.


  #28  
Old March 6th 19, 02:02 PM posted to rec.bicycles.tech
AMuzi
external usenet poster
 
Posts: 10,341
Default Wheel weight

On 3/5/2019 9:55 PM, jbeattie wrote:
On Tuesday, March 5, 2019 at 7:28:17 PM UTC-8, James wrote:
On 6/3/19 12:47 pm, John B. Slocomb wrote:
On Tue, 5 Mar 2019 16:44:03 -0800, "Mark J."
wrote:


I think it's the trigonometry of the computation; the GPS (as I
understand it) measures distance to a collection of satellites whose
positions are well known, then computes location from triangulating the
results. I'm guessing that since most of the satellites are usually not
directly overhead, but rather the line of sight to the satellite is
usually be much closer to tangential to the earth, then very small
errors in the distance-to-satellite computation turn into much larger
errors in the altitude computation.

I think this is why higher-end bike GPS's have a pressure-based
altimeter as well, to correct the fluctuations in the GPS-computed
altitude. I know my Garmin Edge's regularly solicit known altitude
input at the start of a course.


I'm not sure about how accurate GPS really is but back in the day, the
seismic folks had a large "Black Box" that they used to locate their
seismic lines on the chart that they said was accurate to within feet.


Mark is pretty close to the correct reason. Most GPS receivers
intentionally track satellites that are close to the horizon as opposed
to direct overhead. This is so that the X-Y part of the position
information is most accurate, at the expense of less accurate Z position.


The overall accuracy depends greatly on the GPS receiver quality. The
cheap receivers (say $50 ea) may be within a few meters, while expensive
receivers ($500) are 10 times better. If you pay more ($10,000) and
incorporate corrections for atmospheric conditions and such, accuracy
can be better still.

The difference between cheap and expensive is largely down to the
stability of the oscillator used to time signals. The antennas can also
be an expensive part and play a big role in accuracy and reflected
signal rejection.

But... Even cheap GPS receivers are relatively stable over a short
time. They usually produce a position, speed and heading once per
second. The previous position, speed and heading are combined with new
measurements in a special filter, that usually results in better
accuracy than if the measurements were used alone.

The only times I've noticed real problems is when you cycle relatively
fast around tight corners. The GPS position effectively cuts a little
off the corner each time, modelling it as a series of straight lines.
Hence your road speed appears to be slower than it really is and you
appear to accelerate again when the road straightens out.

More expensive GPS receivers can produce calculated position results
more frequently than 1 per second. More powerful processor. More power
consumption. Unlikely to be in a battery operated consumer grade bike
computer.

But I find that they are accurate enough not to miss the magnet and reed
switch.


I have no instrumentation. I like surprises at the end of the ride when I ask my fully instrumented riding buddies how far we went and how much we climbed -- then I round up. No data to prove me wrong. I adopt my son's power data when we're riding together since we're both about the same weight, although he is all muscle and I'm muscle and other things. He gave me a Stages GPS Garmin-ish thing from work, but I haven't put it on my bike. It sends me an e-mail every week reminding me that I haven't ridden any miles. That's super helpful.

-- Jay Beattie.





+1
I don't need to know; it's not why I ride.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971


  #29  
Old March 6th 19, 02:29 PM posted to rec.bicycles.tech
Duane[_2_]
external usenet poster
 
Posts: 319
Default Wheel weight

On 06/03/2019 9:02 a.m., AMuzi wrote:
On 3/5/2019 9:55 PM, jbeattie wrote:
On Tuesday, March 5, 2019 at 7:28:17 PM UTC-8, James wrote:
On 6/3/19 12:47 pm, John B. Slocomb wrote:
On Tue, 5 Mar 2019 16:44:03 -0800, "Mark J."
wrote:


I think it's the trigonometry of the computation; the GPS (as I
understand it) measures distance to a collection of satellites whose
positions are well known, then computes location from triangulating
the
results.┬* I'm guessing that since most of the satellites are
usually not
directly overhead, but rather the line of sight to the satellite is
usually be much closer to tangential to the earth, then very small
errors in the distance-to-satellite computation turn into much larger
errors in the altitude computation.

I think this is why higher-end bike GPS's have a pressure-based
altimeter as well, to correct the fluctuations in the GPS-computed
altitude.┬* I know my Garmin Edge's regularly solicit known altitude
input at the start of a course.


I'm not sure about how accurate GPS really is but back in the day, the
seismic folks had a large "Black Box" that they used to locate their
seismic lines on the chart that they said was accurate to within feet.


Mark is pretty close to the correct reason. Most GPS receivers
intentionally track satellites that are close to the horizon as opposed
to direct overhead.┬* This is so that the X-Y part of the position
information is most accurate, at the expense of less accurate Z
position.


The overall accuracy depends greatly on the GPS receiver quality.┬* The
cheap receivers (say $50 ea) may be within a few meters, while expensive
receivers ($500) are 10 times better.┬* If you pay more ($10,000) and
incorporate corrections for atmospheric conditions and such, accuracy
can be better still.

The difference between cheap and expensive is largely down to the
stability of the oscillator used to time signals.┬* The antennas can also
be an expensive part and play a big role in accuracy and reflected
signal rejection.

But...┬* Even cheap GPS receivers are relatively stable over a short
time.┬* They usually produce a position, speed and heading once per
second.┬* The previous position, speed and heading are combined with new
measurements in a special filter, that usually results in better
accuracy than if the measurements were used alone.

The only times I've noticed real problems is when you cycle relatively
fast around tight corners.┬* The GPS position effectively cuts a little
off the corner each time, modelling it as a series of straight lines.
Hence your road speed appears to be slower than it really is and you
appear to accelerate again when the road straightens out.

More expensive GPS receivers can produce calculated position results
more frequently than 1 per second.┬* More powerful processor.┬* More power
consumption.┬* Unlikely to be in a battery operated consumer grade bike
computer.

But I find that they are accurate enough not to miss the magnet and reed
switch.


I have no instrumentation. I like surprises at the end of the ride
when I ask my fully instrumented riding buddies how far we went and
how much we climbed -- then I round up. No data to prove me wrong. I
adopt my son's power data when we're riding together since we're both
about the same weight, although he is all muscle and I'm muscle and
other things.┬* He gave me a Stages GPS Garmin-ish thing from work, but
I haven't put it on my bike. It sends me an e-mail every week
reminding me that I haven't ridden any miles.┬* That's super helpful.

-- Jay Beattie.





+1
I don't need to know; it's not why I ride.


I, on the other hand, am pretty wired up with my Garmin and Strava and
RideWithGPS. I like the stats to show my progress. I find the GPS helps
me when leading groups on rides that I don't know the route so well.
Less problematic than paper maps.

What's cool about cycling is that we are both happy with what we have.
  #30  
Old March 6th 19, 02:35 PM posted to rec.bicycles.tech
Zen Cycle
external usenet poster
 
Posts: 91
Default Wheel weight

On Tuesday, March 5, 2019 at 10:28:17 PM UTC-5, James wrote:
On 6/3/19 12:47 pm, John B. Slocomb wrote:
On Tue, 5 Mar 2019 16:44:03 -0800, "Mark J."
wrote:


I think it's the trigonometry of the computation; the GPS (as I
understand it) measures distance to a collection of satellites whose
positions are well known, then computes location from triangulating the
results. I'm guessing that since most of the satellites are usually not
directly overhead, but rather the line of sight to the satellite is
usually be much closer to tangential to the earth, then very small
errors in the distance-to-satellite computation turn into much larger
errors in the altitude computation.

I think this is why higher-end bike GPS's have a pressure-based
altimeter as well, to correct the fluctuations in the GPS-computed
altitude. I know my Garmin Edge's regularly solicit known altitude
input at the start of a course.


I'm not sure about how accurate GPS really is but back in the day, the
seismic folks had a large "Black Box" that they used to locate their
seismic lines on the chart that they said was accurate to within feet.


Mark is pretty close to the correct reason. Most GPS receivers
intentionally track satellites that are close to the horizon as opposed
to direct overhead. This is so that the X-Y part of the position
information is most accurate, at the expense of less accurate Z position.


The overall accuracy depends greatly on the GPS receiver quality. The
cheap receivers (say $50 ea) may be within a few meters, while expensive
receivers ($500) are 10 times better. If you pay more ($10,000) and
incorporate corrections for atmospheric conditions and such, accuracy
can be better still.

The difference between cheap and expensive is largely down to the
stability of the oscillator used to time signals. The antennas can also
be an expensive part and play a big role in accuracy and reflected
signal rejection.

But... Even cheap GPS receivers are relatively stable over a short
time. They usually produce a position, speed and heading once per
second. The previous position, speed and heading are combined with new
measurements in a special filter, that usually results in better
accuracy than if the measurements were used alone.

The only times I've noticed real problems is when you cycle relatively
fast around tight corners. The GPS position effectively cuts a little
off the corner each time, modelling it as a series of straight lines.
Hence your road speed appears to be slower than it really is and you
appear to accelerate again when the road straightens out.

More expensive GPS receivers can produce calculated position results
more frequently than 1 per second. More powerful processor. More power
consumption. Unlikely to be in a battery operated consumer grade bike
computer.

But I find that they are accurate enough not to miss the magnet and reed
switch.


There's also the ability to track more than a few satellites. Mathematically you only need three for acquisition, but the more satellites you track, the more accurate your calculation will be. Early and chepaer GPS units would only track three or four, since the smaller microcontrollers can only handle juggling so much data. More elaborate GPS systems will track as many satellites as they can see. A system I worked on recently could log up to 26 at once, though we generally considered 9 to be optimal for the system architecture and the application.

I also remember the cornering issue. I bought a rather nice garmin unit about ten years ago, and was frustrated when I found out the averaging was locked at 5 seconds (if there was a way to decrease the sampling interval, I couldn't find it). That really sucked for logging my local singletrack trails. These days I just use my phone with strava. I get an occasional error but those "lines" you mentioned are exceptionally short.
 




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