#21
|
|||
|
|||
GPS (was: Wheel weight)
On Wed, 6 Mar 2019 00:23:00 +0000 (UTC), Claus Aßmann
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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.Â* 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.Â* 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.Â* 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.Â* The cheap receivers (say $50 ea) may be within a few meters, while expensive receivers ($500) are 10 times better.Â* 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.Â* The antennas can also be an expensive part and play a big role in accuracy and reflected signal rejection. But...Â* Even cheap GPS receivers are relatively stable over a short time.Â* They usually produce a position, speed and heading once per second.Â* 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.Â* 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.Â* More powerful processor.Â* More power consumption.Â* 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
|
|||
|
|||
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
|
|||
|
|||
GPS
On 3/5/2019 8:41 PM, John B. Slocomb wrote:
On Wed, 6 Mar 2019 00:23:00 +0000 (UTC), Claus Aßmann 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
|
|||
|
|||
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 Aßmann 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
|
|||
|
|||
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
|
|||
|
|||
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.Â* 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.Â* 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.Â* 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.Â* The cheap receivers (say $50 ea) may be within a few meters, while expensive receivers ($500) are 10 times better.Â* 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.Â* The antennas can also be an expensive part and play a big role in accuracy and reflected signal rejection. But...Â* Even cheap GPS receivers are relatively stable over a short time.Â* They usually produce a position, speed and heading once per second.Â* 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.Â* 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.Â* More powerful processor.Â* More power consumption.Â* 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.Â* 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.Â* 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
|
|||
|
|||
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. |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Wheel Weight Inexplicably Increases 20 gms In 2 Weeks | Bret Cahill | UK | 2 | August 13th 18 05:59 PM |
Adding weight to a wheel | feel the light | Unicycling | 41 | March 25th 08 08:14 PM |
What happens if you hang a weight from the bottom of a wheel? | [email protected] | Techniques | 16 | September 17th 06 06:42 AM |
Bike weight=Rider weight | Penster | Techniques | 25 | August 14th 06 02:36 AM |
Is body weight equivalent to bicycle weight? | Bruce W.1 | Techniques | 37 | July 27th 05 01:45 AM |