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Are GPS computers accurate?

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Are GPS computers accurate?

Old 11-06-20, 07:40 PM
  #26  
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Originally Posted by Riveting View Post
Speed (avg. and instantaneous) is unimportant anyway, there's too many variables involved that change from ride-to-ride, or hour-to-hour (wind, hill gradient, tire rolling resistance, bike weight, rider weight, clothing aero drag, riding position aero drag, drafting other riders) to make it useful for analysis, except maybe to avoid breaking the speed limit, if that's your thing. Once you come to grips with that, the GPS inaccuracy won't matter to you. If you really care to monitor your performance, I'd recommend getting a torque based power meter, and you'll never look at speed again.
i do use average speed to determine improvement and fitness level. At the beginning of the season, after taking winter off, my average speed for the first week was about 14 MPH. Today riding the same route with an additional 300 climb thrown is, my average was 19.3. I enjoy competing against others and myself. I know that is not for everyone but I get a certain level of satisfaction by increasing my speed, endurance and distance. But even at 65 I still have a defective gene that is achievement and goal oriented. I do stop and enjoy views and wildlife and to admire the wonder at it all.
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Old 11-06-20, 08:15 PM
  #27  
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Originally Posted by CargoDane View Post
You first tried to dismiss other sat systems and whatnot because they weren't in cycle computers, but now you post a link to a frigging watch? Yes, a barometer in a watch is a cheap way of getting altitude data - especially as it probably haven't the space to use that many different GNSS systems.
Yes, that watch (in 2017) uses GPS (not Galileo, Glonass, or anything else), so it can be hard for it to receive enough sat signals to get a precise altitude. If you have a watch/cyclecomputer/whatever that uses more than one system, it will have that many more sats to connect to, improving measurements immensely. 2017 is ages ago in the smartwatch "scene".
D-GPS and WAAS aren't "sat systems".

Talking about units that have to be stationary and use ground-based systems to get accurate data in a discussion about bicycle GPS units make no sense at all.

The Garmin watches don't seem to perform any different than the cycling units.

It seems to be difficult to get more-recent info. But I provided something and you provided nothing.

And, even for the latest units, Garmin rates barometric data over GPS data.


​​

Last edited by njkayaker; 11-07-20 at 07:07 AM.
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Old 11-06-20, 08:25 PM
  #28  
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Originally Posted by CargoDane View Post
Reading comprehension, please.
They are talking about (a decade ago) just how precise GPS survey equipment is. Meaning even then (and without Glonass, Beidou, or Galileo) they could get VERY accurate results.


Coming from someone like you who can't grasp what is being said or read links, nor understand how a GPS tells you where you are, nor what triangulation is, I'm perfectly fine with that.

If you still have problems, here's an easy-to-understand link (and why I talked about "spokes" as a way of illustrating it:

https://www.nationalgeographic.org/p...ulation-sized/




https://www.nationalgeographic.org/p...ulation-sized/
Not triangulation.


https://gisgeography.com/trilaterati...ngulation-gps/
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Old 11-07-20, 04:33 PM
  #29  
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Originally Posted by njkayaker
Talking about units that have to be stationary and use ground-based systems to get accurate data in a discussion about bicycle GPS units make no sense at all.
You posted the link. Not me. And the first article was more than 20 years old, the newest was more than a decade old. Things have moved on since then: We now have Galileo,Beidou, and others in addition to the now very-old "GPS".

As for D-GPS and WAAS. Back then, GPS was SCRAMBLED, so it wasn't as precise as it is now for civil use. So people thought up ways to augment the system to become MORE precise.
In January 2000 President Clinton signed an order to descramble the GPS signal, making for a more precise signal directly from the satellites. That's how old and out-of-date your first reference is.


Originally Posted by njkayaker View Post
I know. I compared it to doing the same with a hand bearing compass, and since simple concepts seemed to be over your heads, I used simplified language to convey the idea.

Originally Posted by njkayaker View Post
D-GPS and WAAS aren't "sat systems".

The Garmin watches don't seem to perform any different than the cycling units.

It seems to be difficult to get more-recent info. But I provided something and you provided nothing.

And, even for the latest units, Garmin rates barometric data over GPS data.

​​
D-GPS and WAAS are add-ons to sat systems, and as such they are part of the sat system even if they use land based positioning/relays to augment the system. And it was done as a patch on the SCRAMBLED GPS, because back then it was SCRAMBLED for civilians, making for much less accuracy.


As for the watch: A barometer is easier, cheaper, and packs into a smaller size than using chips and antennas for Beidou, Glonass, Galileo and so on. THe fact that there is a barometer doesn't mean it is then better at finding your altitude. A barometer is very useful for actually checking the atmospheric pressure and sudden drops in it.

As for me not providng anything: I provided facts and actually explained how the systems work. You provided decades old information that isn't even true of the original system today. Things move on and we get better at technology as time moves on. What you did was akin to providing a medical article that was two decades old and then claiming that things can't possibly have moved on since then. That is not "providing something". That is "fielding old information and presenting it as a current fact".


First you argue that you can't have glonass, galileo etc. in a small cycle computer, and then you go on to argue that a barometer must be better because Garmin watches has a barometer and used one for altitude in 2017.
And this in addition to you posting "support" for the idea that satellite positioning systems cannot be used for altitude. "Support" that in one instance was 20 years old (which even mentioned surveying equipment which was VERY precise), and an article that was a decade old.

Constantly moving the goal posts as you constantly do is not a good look.

Last edited by CargoDane; 11-07-20 at 04:48 PM.
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Old 11-07-20, 04:57 PM
  #30  
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Originally Posted by CargoDane View Post

Constantly moving the goal posts as you constantly do is not a good look.
Don’t be ridiculous.

You mentioned “survey grade” units when people were talking about bicycle GPS units.


Originally Posted by CargoDane View Post
First you argue that you can't have glonass, galileo etc. in a small cycle computer,
????

I never said that.

Last edited by njkayaker; 11-07-20 at 05:09 PM.
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Old 11-07-20, 05:10 PM
  #31  
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Originally Posted by njkayaker View Post
Don’t be ridiculous.

You mentioned “survey grade” units when people were talking about bicycle GPS units.
I mentioned that because it was in your two decades old article writing about equipment from BEFORE GPS was UNSCRAMBLED!

Do you read your own damn "proof"? Sheesh.

Thanks for the head-up, though: You're merely a copy/paster who doesn't read your own damn "evidence" - to the extent that you don't even check the date of it.
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Old 11-07-20, 05:16 PM
  #32  
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Originally Posted by njkayaker View Post


I never said that.
You said that GPS was bad for altitude. THen moved on to claim that small cycle computers couldn't be as good as pro gear and hence altitude was still bad.

You then moved on to talk about how small cycle computers was, and then moved on from there to post about watches. It's all up-thread.
But whom am I kidding. You only copy/paste stuff - so old in fact, that your first piece of "evidence" was from before GPS was unscrambled and the later article was from when Galileo was but a proposal and Glonass a dream.

You merely copy/paste old stuff that is not relevant today at all and claim that as "providing something". Yes, you provide "something": So out of date information that it is directly misleading.


I'm done with you.
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Old 11-07-20, 07:03 PM
  #33  
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Three of us did a defined ride of 48.8 miles. My old fashioned cycle computer pegged the ride at 48.8 miles. One guy (with a GPS) had the ride at 49.31 miles. The other guy with GPS was at 48.39. The elevation gains were not that different and were within 10-30 feet. To me, GPS is close enough.
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Old 11-08-20, 08:29 AM
  #34  
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I use a cadence/speed sensor with my Edge 530. It used the GPS to help determine the wheel circumference. I honestly do not know if it uses the speed sensor or not during outdoor rides.

Anyone know the answer to that?
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Old 11-08-20, 09:16 AM
  #35  
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I have to say I don't really care much about speed or distance. Do they ask you if you want to calibrate the wheel sensor?
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Old 11-08-20, 10:20 AM
  #36  
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Originally Posted by rsbob View Post
Good to know. Thanks guys. I recently bought a Swiss made Sigma computer which is non-GPS but does provide accurate speed, distance, time which I am happy with. Since I have Strava on my phone I can get all the info I need, even though Strava suffers from the same speed/Ave speed issues.
Originally Posted by rsbob View Post
i do use average speed to determine improvement and fitness level. At the beginning of the season, after taking winter off, my average speed for the first week was about 14 MPH. Today riding the same route with an additional 300 climb thrown is, my average was 19.3. I enjoy competing against others and myself. I know that is not for everyone but I get a certain level of satisfaction by increasing my speed, endurance and distance. But even at 65 I still have a defective gene that is achievement and goal oriented. I do stop and enjoy views and wildlife and to admire the wonder at it all.
If you had simply added a speed sensor (wheel sensor) and your GPS device is able to use that info, then it'll fix your issue. Every cycling GPS I've looked at defaults to using the wheel sensor info when available. Of course if a phone app is your GPS device, then there have been exceptions.

With your Sigma that I guess is a non GPS device, you've solved that for when you are on the bike because I assume it's one that has some sort of wheel sensor. But your data uploaded to Strava and other places via what ever GPS device you use won't have that info. But they could if you'd get a wheel sensor that is compatible with your GPS device.
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Old 11-08-20, 10:40 AM
  #37  
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Originally Posted by Iride01 View Post
If you had simply added a speed sensor (wheel sensor) and your GPS device is able to use that info, then it'll fix your issue. Every cycling GPS I've looked at defaults to using the wheel sensor info when available. Of course if a phone app is your GPS device, then there have been exceptions.

With your Sigma that I guess is a non GPS device, you've solved that for when you are on the bike because I assume it's one that has some sort of wheel sensor. But your data uploaded to Strava and other places via what ever GPS device you use won't have that info. But they could if you'd get a wheel sensor that is compatible with your GPS device.
certainly appreciate your thoughts. The right thing to do is to get what you suggest, but I can only throw so much money at a non-problem. Maybe Santa will help me out. Thanks for your reply.
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Old 11-08-20, 10:52 AM
  #38  
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Originally Posted by CargoDane View Post
As for the watch: A barometer is easier, cheaper, and packs into a smaller size than using chips and antennas for Beidou, Glonass, Galileo and so on. THe fact that there is a barometer doesn't mean it is then better at finding your altitude. A barometer is very useful for actually checking the atmospheric pressure and sudden drops in it.
More stuff you have no idea about. The Garmin watches and cycling units have supported GLONASS and Galileo for a while.

Garmin rates barometric data as better.

Originally Posted by CargoDane View Post
I mentioned that because it was in your two decades old article writing about equipment from BEFORE GPS was UNSCRAMBLED!

Do you read your own damn "proof"? Sheesh.

Thanks for the head-up, though: You're merely a copy/paster who doesn't read your own damn "evidence" - to the extent that you don't even check the date of it.
Still ridiculous.

You didn't provide any links (until late). If you don't like the links, provide better ones rather than blathering incoherently.

Originally Posted by CargoDane View Post
First you argue that you can't have glonass, galileo etc. in a small cycle computer,
I never said this, Talk about moving goal posts!

Originally Posted by CargoDane View Post
You said that GPS was bad for altitude.
Garmin rates barometric data as better.

Originally Posted by CargoDane View Post
You said that GPS was bad for altitude. THen moved on to claim that small cycle computers couldn't be as good as pro gear and hence altitude was still bad.
???? This is just clueless.

Small devices aren't as good as "survey grade" devices that use D-GPS and WAAS.

https://mapasyst.extension.org/what-...vey-grade-gps/

Last edited by njkayaker; 11-08-20 at 11:15 AM.
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Old 11-08-20, 11:34 AM
  #39  
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Originally Posted by njkayaker View Post
More stuff you have no idea about. The Garmin watches and cycling units have supported GLONASS and Galileo for a while.

Garmin rates barometric data as better.



Still ridiculous.

You didn't provide any links (until late). If you don't like the links, provide better ones rather than blathering incoherently.


I never said this, Talk about moving goal posts!


Garmin rates barometric data as better.


???? This is just clueless.

Small devices aren't as good as "survey grade" devices that use D-GPS and WAAS.

https://mapasyst.extension.org/what-...vey-grade-gps/
They obviously aren't. Hence my point that going to even smaller than the cycling computer is ridiculous. That article which referenced that was from BEFORE GPS WAS UNSCRAMBLED. SInce then, not only is GPS unscrambled, but there are more precise sat navigation systems available, plus better hardware when it comes to handheld devices and smaller.


What is so difficult to understand here? You don't read the articles you post, you don't even realise that you posted an article from before GPS was unscrambled twenty years, you fail to understand your own arguments, and you fail to understand technology.
No one can keep at it and continuously move the goalposts like you do and yet still be considered honest. Especially not when reality has been pointed out to you so many times. You are now on my ignore list.

I forgot, since you still refuse to face facts:
Before GPS was unscrambled (as at the time of writing that USCG article), the precision was "within 100 metres" (330ft) in ideal conditions. Obviously, that isn't even close enough for good altidude. Since then, not only has GPS gotten better, there are more precise sat systems available, AND the receivers of today - regardless of size - receives the signal from many more sats than they did in 2000 and before.

Last edited by CargoDane; 11-08-20 at 11:46 AM.
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Old 11-08-20, 11:55 AM
  #40  
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Originally Posted by CargoDane View Post
doing the same with a hand bearing compass, and since simple concepts seemed to be over your heads, I used simplified language to convey the idea.
You posted a link you knew was wrong?

Originally Posted by CargoDane View Post
You merely copy/paste old stuff that is not relevant today at all and claim that as "providing something". Yes, you provide "something": So out of date information that it is directly misleading.
??? You posted something you knew wasn't correct!


Originally Posted by CargoDane View Post
Before GPS was unscrambled (as at the time of writing that USCG article), the precision was "within 100 metres" (330ft) in ideal conditions. Obviously, that isn't even close enough for good altidude. Since then, not only has GPS gotten better, there are more precise sat systems available, AND the receivers of today - regardless of size - receives the signal from many more sats than they did in 2000 and before.
"Selective availability" was turned off 20 years ago. No one is talking about that.

Garmin rates barometric data as better right now for current units (ones that use can use GLONASS and Galileo).

People here are talking about cycling GPS units.

The performance of "survey grade" units, which use other things (that small devices don't use) is irrelevant and "moving the goal posts".

​​​​​
Originally Posted by CargoDane View Post
I'm done with you.
Yet, here you are, still being ridiculous.

Last edited by njkayaker; 11-08-20 at 12:07 PM.
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Old 11-08-20, 12:35 PM
  #41  
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No, I didn't post a link that was wrong in any way. I was not the tosser who posted the link with the Survery grade info in it (the USCG article). You posted that.


To explain in easy terms I made a comparison to using a hand-bearing compass. Hence I used the terms you use when use a hand bearing compass. It made everything simpler. Yet you still failed to grasp the simple concept.

And, yes, Garmin rates their own products as using the best technology ever. What a surprise. It wouldn't actually sell units using cheap hardware if it said they used old and cheap technology to improve their margins, now would it.

As for your selective availability comment. You are talking about the unscrambling of the GPS signal. Don't pretend it is something else and as if it was something you knew. Don't pretend you actually know something, because it is obvious you know absolutely nothing about GPS:

https://www.wired.com/2000/05/clinto...s-gps-signals/

https://www.nytimes.com/2000/05/02/t...-accurate.html

https://www.sciencemag.org/news/2000...es-gps-signals
And ten years after (2010):
https://www.cnet.com/news/celebratin...or-the-masses/

It is obvious you looked it up, then looked for something that would somehow disprove that it was unscrambled in the year 2000. It's another word for the implementation of scrambling of the GPS signal so only military units could use the higher precision signal.

Let me repeat: At the time of writing that article you posted as "evidence", the best precision GPS could muster was 100 metres in ideal conditions. The reason was that the signal was scrambled so non-military units couldn't use it precisely enough for nefarious reasons.
Another titbit of information you will continue to ignore is that the reason the US military stopped the scrambling of the signal was that technology had caught up to it, so scrambling it was pointless. Hence Clinton signing an order in May 2000 to stop the scrambling of the GPS signal.

But you're right I kept coming back to your posts, because I hadn't also unsubscribed. That was idiotic of me. Now I will both have unsubscribed as well as having you and your posts on ignore.
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Old 11-08-20, 01:36 PM
  #42  
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Originally Posted by tntyz View Post
I use a cadence/speed sensor with my Edge 530. It used the GPS to help determine the wheel circumference. I honestly do not know if it uses the speed sensor or not during outdoor rides.

Anyone know the answer to that?
If the sensor was correctly paired the first time with the GPS unit, it should connect every time the sensor starts, which is when you begin rotating the wheel. You can tell in the initial system page (swipe down the screen, or whatever you do on a button unit). You will see what sensors are running.

On a a Garmin, if a sensor is active, the speed and distance data will be used in place of the GPS data.
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Old 11-09-20, 06:58 PM
  #43  
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For reference, aircraft explicitly DO NOT use GPS for altitude. Aircraft altitude is always barometric primary. An ILS approach can utilize GPS but the primary altitude data will be from a barometer.
A standard GPS unit has a vertical accuracy of about 10m vs about 3m for lat/long. Barometric altitude is more sensitive for small changes in elevation, but the base elevation can vary based on the overall weather. For aircraft, take-off and landing, altitude is based on a FAA-certified barometer at the airport. Above 18k ft all aircraft are flying at pressure altitude of 29.92inHg, the actual altitude will vary according weather, but the important part is that all aircraft vary TOGETHER. The most important thing is that aircraft maintain 1000ft vertical separation.
For a bicycle, barometric altitude will give you a better estimate of elevation gain & loss, but potentially less accurate for the absolute altitude, such as the top of a hill.

A Garmin unit will use wheel speed sensors for speed and distance, but it calibrates that number very accurately using GPS. The traditional method of wheel size calibration is using a roll-out test and a tape measure. The Garmin unit does the same thing, but all it needs is basically a 1km long section of straight road.

The next generation of GPS sensors that should be hitting the market relatively soon will have ~cm level accuracy. Those sensors are based on using both L1 & L5 GPS bands, and carrier phase tracking. This technology already exists in survey grade GPS units and that's why the performance isn't comparable to a bicycle unit. Autonomous vehicles really need cm level GPS accuracy to enable lane tracking.
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Old 11-09-20, 07:07 PM
  #44  
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Originally Posted by CargoDane View Post
No, I didn't post a link that was wrong in any way..
And you have still not provided any current links supporting what you said!

You just keep talking about irrelevant old stuff.

Originally Posted by CargoDane View Post
And, yes, Garmin rates their own products as using the best technology ever. What a surprise. It wouldn't actually sell units using cheap hardware if it said they used old and cheap technology to improve their margins, now would it.
This is stupid.

Their margins are less using barometers!

If GPS was better for cycling units, Garmin would save money by not adding barometers!

And they rate it as better while saving that money!


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Old 11-09-20, 07:21 PM
  #45  
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Originally Posted by gsa103 View Post
...
A standard GPS unit has a vertical accuracy of about 10m vs about 3m for lat/long. Barometric altitude is more sensitive for small changes in elevation, ...
For a bicycle, barometric altitude will give you a better estimate of elevation gain & loss, but potentially less accurate for the absolute altitude, such as the top of a hill.
100 feet is a much larger "noise" signal on successive elevation measurements than 10 feet is on successive horizontal measurements.

The impact of the error on gain is also larger than on horizontal displacement given that the gain is based on much smaller differences. It's even worse because the vertical speed is much lower, generally, than horizontal.

GPS might be ok for casual absolute elevation. But not many cyclists are interested in that.

Last edited by njkayaker; 11-09-20 at 07:30 PM.
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Old 11-09-20, 07:42 PM
  #46  
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Originally Posted by gsa103 View Post
For reference, aircraft explicitly DO NOT use GPS for altitude. Aircraft altitude is always barometric primary. An ILS approach can utilize GPS but the primary altitude data will be from a barometer.
A standard GPS unit has a vertical accuracy of about 10m vs about 3m for lat/long. Barometric altitude is more sensitive for small changes in elevation, but the base elevation can vary based on the overall weather. For aircraft, take-off and landing, altitude is based on a FAA-certified barometer at the airport. Above 18k ft all aircraft are flying at pressure altitude of 29.92inHg, the actual altitude will vary according weather, but the important part is that all aircraft vary TOGETHER. The most important thing is that aircraft maintain 1000ft vertical separation.
For a bicycle, barometric altitude will give you a better estimate of elevation gain & loss, but potentially less accurate for the absolute altitude, such as the top of a hill.

A Garmin unit will use wheel speed sensors for speed and distance, but it calibrates that number very accurately using GPS. The traditional method of wheel size calibration is using a roll-out test and a tape measure. The Garmin unit does the same thing, but all it needs is basically a 1km long section of straight road.

The next generation of GPS sensors that should be hitting the market relatively soon will have ~cm level accuracy. Those sensors are based on using both L1 & L5 GPS bands, and carrier phase tracking. This technology already exists in survey grade GPS units and that's why the performance isn't comparable to a bicycle unit. Autonomous vehicles really need cm level GPS accuracy to enable lane tracking.
1: Just to emphasize what you're saying: So an airplane checks (basically zeroes the barometer) with the airport's barometer before landing in order to make the barometer reading accurate, otherwise it isn't good enough. A barometer you set before you took off isn't precise enough. Bicycle computers do not have the luxury of having airports to zero their barometer readings everywhere.

What you just explained doesn't actually prove what you think it proves. Or what Njkayaker thinks it proves.

2: GPS is but one of several GNSS systems ("GNS Systems" to be correct) out there - several of which is already in use - including used by bicycle computers and even smart watches. The reason GPS is upping their game is that they are being outclassed by other GNS systems.

3: Garmin may default to a wheel-sensor, but most people don't actually have that installed with their bicycle computer - Garmin or otherwise - unless it is a non-GNSS unit.

Last edited by CargoDane; 11-09-20 at 07:46 PM.
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Old 11-09-20, 07:48 PM
  #47  
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Originally Posted by CargoDane View Post
1: Just to emphasize what you're saying: So an airplane checks (basically zeroes the barometer) with the airport's barometer in order to make the barometer reading accurate, otherwise it isn't good enough. A barometer you set before you took off isn't precise enough. Bicycle computers do not have the luxury of having airports to zero their barometer readings everywhere. What you just explained doesn't actually prove what you think it proves. Or what Njkayaker thinks it proves.
More stupidity!

You don't really need to zero barometers for elevation gain.

If GPS was better than barometric measurements for airplanes, pilots wouldn't use barometers!

Originally Posted by CargoDane View Post
3: Garmin may default to a wheel-sensor, but most people don't actually have that installed with their bicycle computer - Garmin or otherwise - unless it is a non-GNSS unit.
The main use of wheel sensors is for accurate instantaneous speed. At cycling speeds, GPS is not very good for that. If you aren't interested in accurate instantaneous speed, you really don't need a wheel sensor.


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Last edited by njkayaker; 11-09-20 at 07:56 PM.
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Old 11-09-20, 07:49 PM
  #48  
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Read what he is actually saying. It seems he hasn't noticed himself.
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Old 11-09-20, 07:55 PM
  #49  
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Let me repeat: They are ONLY able to use the barometer if they compare it to the barometer reading at the airport they're about to land at.

If they don't, their barometer is useless. Atmospheric pressures changes constantly. Can you check somewhere with your bicycle barometer to check with an already known and absolute height? No, of course you can't. Hence your frigging barometer is not accurate.

The "check" with the airport is done so they know what the pressure is at that precise location. They know exactly what absolute height that airport barometer is at, and then it's simply a matter of subtraction/addition to get your height. Does your bicycle computer comunicate with a VERY nearby known-altitude barometer? Of course not.
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Old 11-09-20, 08:05 PM
  #50  
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Originally Posted by CargoDane View Post
Does your bicycle computer comunicate with a VERY nearby known-altitude barometer? Of course not.
It doesn't need to, to measure elevation gain.
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