How accurate are GPS speed-readings?

[Ext User(Michael.Pasturi@gmail.com)]

More accurate than a std. car speedo?

M/P

[Ext User(John_H)]

Michael.Pasturi@gmail.com wrote:

>More accurate than a std. car speedo?

If the antenna is seeing an open sky a typical hand held GPS receiver
is accurate to 1 kph (or better) at steady speed, but not particularly
accurate on acceleration or deceleration.

They calculate speed from distance/time with a typical sampling rate
of 2 readings per second... the reading is the average speed over a
distance which relates back to the sampling rate. If the reading is
steady over a few samplings (read seconds) then it will be accurate to
the best of the unit's ability.

Differential GPS equipment has a much higher sampling rate and vastly
better positional accuracy and hence arrives at a steady reading in a
much shorter time. When I've run both side by side with roof mounted
antennae, which is often, the steady speed readings are near as damnit
to the same (typically within 0.2 kph).

Most modern car speedos I've checked have been within 2 kph of the GPS
reading at 100 kph when running the standard tyre size, and almost
invariably read fast. Tyre wear doesn't affect the speedo accuracy by
nearly as much as tread depth calculations would indicate (because the
effective circumference is below the tread). Tyre pressure and load
has no measurable effect (within the normal ranges).

To sum up... modern speedos are reasonably accurate under most
conditions but they're not nearly as good as GPS at a steady speed.

--
John H

[Ext User(mf1@project1221.com中�人民共和国)]

On Apr 21, 11:10Â*am, John_H <john4...@inbox.com> wrote:
> Michael.Past...@gmail.com wrote:
> >More accurate than a std. car speedo?
>
> If the antenna is seeing an open sky a typical hand held GPS receiver
> is accurate to 1 kph (or better) at steady speed, but not particularly
> accurate on acceleration or deceleration.
>
> They calculate speed from distance/time with a typical sampling rate
> of 2 readings per second... the reading is the average speed over a
> distance which relates back to the sampling rate. Â*If the reading is
> steady over a few samplings (read seconds) then it will be accurate to
> the best of the unit's ability.
>
> Differential GPS equipment has a much higher sampling rate and vastly
> better positional accuracy and hence arrives at a steady reading in a
> much shorter time. Â*When I've run both side by side with roof mounted
> antennae, which is often, the steady speed readings are near as damnit
> to the same (typically within 0.2 kph).
>
> Most modern car speedos I've checked have been within 2 kph of the GPS
> reading at 100 kph when running the standard tyre size, and almost
> invariably read fast. Â*Tyre wear doesn't affect the speedo accuracy by
> nearly as much as tread depth calculations would indicate (because the
> effective circumference is below the tread). Â*Tyre pressure and load
> has no measurable effect (within the normal ranges).
>
> To sum up... modern speedos are reasonably accurate under most
> conditions but they're not nearly as good as GPS at a steady speed.
>
> --
> John H

Roger and thanks for that J/H, no doubt 100's of websites with this,
just need to Google it...


M/P

[Ext User(Daryl Walford)]

Michael.Pasturi@gmail.com wrote:
> More accurate than a std. car speedo?
>

The Magellan GPS I have claims to accurate to within 1% and I have no
reason to doubt their claim.
Most car speedo's read high, some by as much as 10kph, my Hilux reads
5kph higher than actual but the error increases as speed increases.


Daryl

[Ext User(Scotty)]

"Daryl Walford" <user@example.net> wrote in message
news:132isg3m7aefhc5@corp.supernews.com...
> Michael.Pasturi@gmail.com wrote:
>> More accurate than a std. car speedo?
>>
>
> The Magellan GPS I have claims to accurate to within 1% and I have no
> reason to doubt their claim.
> Most car speedo's read high, some by as much as 10kph, my Hilux reads 5kph
> higher than actual but the error increases as speed increases.
>
>
> Daryl

The speed readings through the SEQld tolls always read higher than my
speedo, is this due to wanting to slow people down even more or is the
speedo out that badly on the Rodeo?

Drive through at 60 it reads 64 drive through at 40 it reads 43, Im not yet
game to try for an over 100 reading in case they have a recorder running
there, it is etoll after all.

[Ext User(rebel)]

On Sat, 21 Apr 2007 11:10:14 +1000, John_H <john4721@inbox.com> wrote:

>Michael.Pasturi@gmail.com wrote:
>
>>More accurate than a std. car speedo?
>
>If the antenna is seeing an open sky a typical hand held GPS receiver
>is accurate to 1 kph (or better) at steady speed, but not particularly
>accurate on acceleration or deceleration.
>
>They calculate speed from distance/time

err, no they don't. They use doppler shift. If you doubt me, go ask in one of
the GPS groups and any number of the residents will point you to authoritative
sources.

Having said that, they are typically better than 1kph at constant speed. As
most speedos whcih comply with the AS (+10/-0% IRC) are 3 to 5% fast, the GPS
wins hands down.

[Ext User(Bernd Felsche)]

rebel <me@privacy.net> wrote:
>On Sat, 21 Apr 2007 11:10:14 +1000, John_H <john4721@inbox.com> wrote:
>>Michael.Pasturi@gmail.com wrote:

>>>More accurate than a std. car speedo?

>>If the antenna is seeing an open sky a typical hand held GPS receiver
>>is accurate to 1 kph (or better) at steady speed, but not particularly
>>accurate on acceleration or deceleration.

>>They calculate speed from distance/time

>err, no they don't. They use doppler shift. If you doubt me, go
>ask in one of the GPS groups and any number of the residents will
>point you to authoritative sources.

>Having said that, they are typically better than 1kph at constant
>speed. As most speedos whcih comply with the AS (+10/-0% IRC) are
>3 to 5% fast, the GPS wins hands down.

Unfortunately; they have "glitches" when passing under bridges,
power lines, etc. I've seen this "add" about 5km/h at constant speed
while passing under a flyover. The indication becomes sensible after
the trouble-spot.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | The object of life is not to be on the side of
X against HTML mail | the majority but to escape finding oneself in
/ \ and postings | the ranks of the insane. -- Marcus Aurelius

[Ext User(Kev)]

Scotty wrote:

> "Daryl Walford" <user@example.net> wrote in message
> news:132isg3m7aefhc5@corp.supernews.com...
>
>>Michael.Pasturi@gmail.com wrote:
>>
>>>More accurate than a std. car speedo?
>>>
>>
>>The Magellan GPS I have claims to accurate to within 1% and I have no
>>reason to doubt their claim.
>>Most car speedo's read high, some by as much as 10kph, my Hilux reads 5kph
>>higher than actual but the error increases as speed increases.
>>
>>
>>Daryl
>
>
> The speed readings through the SEQld tolls always read higher than my
> speedo, is this due to wanting to slow people down even more or is the
> speedo out that badly on the Rodeo?
>
> Drive through at 60 it reads 64 drive through at 40 it reads 43, Im not yet
> game to try for an over 100 reading in case they have a recorder running
> there, it is etoll after all.
>
>


You are correct Scotty
and it's the Kuruby toll speed display that is showing about 4kph above
you actual speed(gateway bridge was usually right on)

I've been through at 80 and it was showing 86
and no matter what vehicle I an in it's always the same

Both my cruiser and Ford were reading about 4kph fast with standard
tyres, the ford with 245/45/17 tyres seem to be the same as the standard
16s that were fitted, the Cruiser now has 285/75/16(33inch)instead of
the 265/70/16(31inch) and the speedo is now reading spot on with my GPS

and my boat(very old 4.2m tinny with very old 35hp Johnson) does
43kph(23knots) flat out

Kev

[Ext User(John Hudson)]

"Kev" <kevcat@optunet.com.au> wrote in message
news:462a182a$0$16553$afc38c87@news.optusnet.com.a u...
> and it's the Kuruby toll speed display that is showing about 4kph above
> you actual speed(gateway bridge was usually right on)
>
> I've been through at 80 and it was showing 86
> and no matter what vehicle I an in it's always the same
> Kev

Isn't the speed limit 60 kph?

http://svc002.bne201p.server-web.com/www/index.aspx?itemID=112

[Ext User(Kev)]

John Hudson wrote:

> "Kev" <kevcat@optunet.com.au> wrote in message
> news:462a182a$0$16553$afc38c87@news.optusnet.com.a u...
>
>>and it's the Kuruby toll speed display that is showing about 4kph above
>>you actual speed(gateway bridge was usually right on)
>>
>>I've been through at 80 and it was showing 86
>>and no matter what vehicle I an in it's always the same
>>Kev
>
>
> Isn't the speed limit 60 kph?
>
> http://svc002.bne201p.server-web.com/www/index.aspx?itemID=112
>
>


yes
your point?


Kev

[Ext User(Scotty)]

"Kev" <kevcat@optunet.com.au> wrote in message
news:462a182a$0$16553$afc38c87@news.optusnet.com.a u...
> Scotty wrote:
>
>> "Daryl Walford" <user@example.net> wrote in message
>> news:132isg3m7aefhc5@corp.supernews.com...
>>
>>>Michael.Pasturi@gmail.com wrote:
>>>
>>>>More accurate than a std. car speedo?
>>>>
>>>
>>>The Magellan GPS I have claims to accurate to within 1% and I have no
>>>reason to doubt their claim.
>>>Most car speedo's read high, some by as much as 10kph, my Hilux reads
>>>5kph higher than actual but the error increases as speed increases.
>>>
>>>
>>>Daryl
>>
>>
>> The speed readings through the SEQld tolls always read higher than my
>> speedo, is this due to wanting to slow people down even more or is the
>> speedo out that badly on the Rodeo?
>>
>> Drive through at 60 it reads 64 drive through at 40 it reads 43, Im not
>> yet game to try for an over 100 reading in case they have a recorder
>> running there, it is etoll after all.
>
>
> You are correct Scotty
> and it's the Kuruby toll speed display that is showing about 4kph above
> you actual speed(gateway bridge was usually right on)
>
> I've been through at 80 and it was showing 86
> and no matter what vehicle I an in it's always the same
>
> Both my cruiser and Ford were reading about 4kph fast with standard tyres,
> the ford with 245/45/17 tyres seem to be the same as the standard 16s that
> were fitted, the Cruiser now has 285/75/16(33inch)instead of the
> 265/70/16(31inch) and the speedo is now reading spot on with my GPS
>
> and my boat(very old 4.2m tinny with very old 35hp Johnson) does
> 43kph(23knots) flat out
>
> Kev

Well Kev your obviously keener to go through at higher speeds than me, can
you try my theory out a bit better and try, at say, 200Kmhr? Just to test
this theory for me? ;o)

Either that or I'll just have to steal a 200SX or STi Impreza to try myself.
Only to prove that its out of course.

[Ext User(John_H)]

rebel wrote:
>On Sat, 21 Apr 2007 11:10:14 +1000, John_H <john4721@inbox.com> wrote:
>>
>>They calculate speed from distance/time
>
>err, no they don't. They use doppler shift. If you doubt me, go ask in one of
>the GPS groups and any number of the residents will point you to authoritative
>sources.

I've seen the claims, and Doppler may well be involved somewhere in
the system, but the indicated speed is certainly involves averaging
over a time period. Whether it's averaging distance/time or the last
20 speed readings obtained by Doppler shift is rather a moot point as
the result will be exactly the same!

I'm happy to accept distance/time as a practical explanation of the
readout behaviour. :)

--
John H

[Ext User(John Hudson)]

"Kev" <kevcat@optunet.com.au> wrote in message
news:462a38d3$0$16553$afc38c87@news.optusnet.com.a u...
> John Hudson wrote:
>
>> "Kev" <kevcat@optunet.com.au> wrote in message
>> news:462a182a$0$16553$afc38c87@news.optusnet.com.a u...
>>
>>>and it's the Kuruby toll speed display that is showing about 4kph above
>>>you actual speed(gateway bridge was usually right on)
>>>
>>>I've been through at 80 and it was showing 86
>>>and no matter what vehicle I an in it's always the same
>>>Kev
>>
>>
>> Isn't the speed limit 60 kph?
>>
>> http://svc002.bne201p.server-web.com/www/index.aspx?itemID=112
>
>
> yes
> your point?
>
>
> Kev

Going 20 kph over the limit when Big Brother is watching might not be a good
idea. I'm beginning to sound like Toby here, but they know who you are and
how fast you're going, next they'll just add the fine to your e-toll
account. The toll collectors are getting a bit pissed off by people
speeding thru their workplace and who wouldn't? Try not to hit me up the
arse when I slow to 60.

huddo

[Ext User(Bernd Felsche)]

John_H <john4721@inbox.com> wrote:
>rebel wrote:
>>On Sat, 21 Apr 2007 11:10:14 +1000, John_H <john4721@inbox.com> wrote:

>>>They calculate speed from distance/time

>>err, no they don't. They use doppler shift. If you doubt me, go
>>ask in one of the GPS groups and any number of the residents will
>>point you to authoritative sources.

>I've seen the claims, and Doppler may well be involved somewhere in
>the system, but the indicated speed is certainly involves averaging
>over a time period. Whether it's averaging distance/time or the last
>20 speed readings obtained by Doppler shift is rather a moot point as
>the result will be exactly the same!

>I'm happy to accept distance/time as a practical explanation of the
>readout behaviour. :)

The "dithering" in positional data makes the resulting speed readout
"fantastic" over short distances. It works better if the position
samples compared for average speed are significantly further apart
than the postional resolution.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | The object of life is not to be on the side of
X against HTML mail | the majority but to escape finding oneself in
/ \ and postings | the ranks of the insane. -- Marcus Aurelius

[Ext User(John_H)]

Bernd Felsche wrote:
>John_H <john4721@inbox.com> wrote:
>
>>I'm happy to accept distance/time as a practical explanation of the
>>readout behaviour. :)
>
>The "dithering" in positional data makes the resulting speed readout
>"fantastic" over short distances. It works better if the position
>samples compared for average speed are significantly further apart
>than the postional resolution.

Dithering doesn't seem to be a major issue since selective access was
removed (in 2000 IIRC). I've got DGPS equipment, using an Omnistar
signal, running alongside a permanently mounted 'handheld' unit and
the speed readings are so close to the same it doesn't matter... but
not so the positional data.

Nor is the positional data nearly as variable as some might suppose.
A significant part of the positional error is 'bias" error, which
doesn't change in the short term... ie it doesn't give the correct
position but the incorrect position doesn't change... hence the speed
readout isn't adversely affected.

If you use a handheld GPS to measure an enclosed area, as I often do,
the result will nearly always be identical to DGPS (for the same
reason). Measure it twice with a handheld, and if the result doesn't
change it will be correct... or at least the odds of it not being
correct will be extremely large. The theoretical error, based on
positional accuracy, is several percent... the practical error is more
like 0.1%.

--
John H

[Ext User(the_dawggie)]

On Apr 21, 11:10 am, John_H <john4...@inbox.com> wrote:
> Michael.Past...@gmail.com wrote:
> >More accurate than a std. car speedo?

Mine fairly much matches the speedo. Could be reading
a tad fast, meh - you get that fitting 31 inch tyres and changing over
the speedo drive cog.

> If the antenna is seeing an open sky a typical hand held GPS receiver
> is accurate to 1 kph (or better) at steady speed, but not particularly
> accurate on acceleration or deceleration.

Lag time between readings.

[Ext User(John_H)]

the_dawggie wrote:
>On Apr 21, 11:10 am, John_H <john4...@inbox.com> wrote:
>
>> If the antenna is seeing an open sky a typical hand held GPS receiver
>> is accurate to 1 kph (or better) at steady speed, but not particularly
>> accurate on acceleration or deceleration.
>
>Lag time between readings.

Typical sampling rate for a handheld is 2 readings per second (usually
stated in the specifications). Lag time for a steady reading is much
longer than that... you can also have a situation (at low speeds)
where you get a declining reading after you've stopped.

I'll put my money on averaging being involved. :)

--
John H

[Ext User(Kev)]

John Hudson wrote:

> "Kev" <kevcat@optunet.com.au> wrote in message
> news:462a38d3$0$16553$afc38c87@news.optusnet.com.a u...
>
>>John Hudson wrote:
>>
>>
>>>"Kev" <kevcat@optunet.com.au> wrote in message
>>>news:462a182a$0$16553$afc38c87@news.optusnet.com.a u...
>>>
>>>
>>>>and it's the Kuruby toll speed display that is showing about 4kph above
>>>>you actual speed(gateway bridge was usually right on)
>>>>
>>>>I've been through at 80 and it was showing 86
>>>>and no matter what vehicle I an in it's always the same
>>>>Kev
>>>
>>>
>>>Isn't the speed limit 60 kph?
>>>
>>>http://svc002.bne201p.server-web.com/www/index.aspx?itemID=112
>>
>>
>>yes
>>your point?
>>
>>
>>Kev
>
>
> Going 20 kph over the limit when Big Brother is watching might not be a good
> idea. I'm beginning to sound like Toby here, but they know who you are and
> how fast you're going, next they'll just add the fine to your e-toll
> account. The toll collectors are getting a bit pissed off by people
> speeding thru their workplace and who wouldn't? Try not to hit me up the
> arse when I slow to 60.


Well ya see only the Police can fine me for speeding not the Toll
collectors and there are no fixed speed cameras in QLD as yet(though I'm
sure they will be coming soon)
now you may or may not be aware that the Etoll gates on the logan and
Gateway have seperate lanes divided by concrete barriers extending about
50m out
there are no toll collectors in the Etoll lanes and no boomgates
and the limit is 60 because you have to merge with traffic leaving the
non-etoll gates about 100m past the plaza

and at midnight when your the only vehicle on the road and the sole
tollie is in the far left lane booth reading a paper doing 80 is not
such a big deal, just have to make sure the evil knevil isn't waiting
for ya on the other side


Kev

[Ext User(Kev)]

John_H wrote:

> rebel wrote:
>
>>On Sat, 21 Apr 2007 11:10:14 +1000, John_H <john4721@inbox.com> wrote:
>>
>>>They calculate speed from distance/time
>>
>>err, no they don't. They use doppler shift. If you doubt me, go ask in one of
>>the GPS groups and any number of the residents will point you to authoritative
>>sources.
>
>
> I've seen the claims, and Doppler may well be involved somewhere in
> the system, but the indicated speed is certainly involves averaging
> over a time period. Whether it's averaging distance/time or the last
> 20 speed readings obtained by Doppler shift is rather a moot point as
> the result will be exactly the same!
>
> I'm happy to accept distance/time as a practical explanation of the
> readout behaviour. :)
>



so instead of using a simple time v distance calculator my old Garmin
GPS12 uses Doppler to give me a speed reading?

Somehow I doubt that


Kev

[Ext User(ReSiN8oR)]

John_H wrote:
> Bernd Felsche wrote:
>
>>John_H <john4721@inbox.com> wrote:
>>
>>
>>>I'm happy to accept distance/time as a practical explanation of the
>>>readout behaviour. :)
>>
>>The "dithering" in positional data makes the resulting speed readout
>>"fantastic" over short distances. It works better if the position
>>samples compared for average speed are significantly further apart
>>than the postional resolution.
>
>
> Dithering doesn't seem to be a major issue since selective access was
> removed (in 2000 IIRC). I've got DGPS equipment, using an Omnistar
> signal, running alongside a permanently mounted 'handheld' unit and
> the speed readings are so close to the same it doesn't matter... but
> not so the positional data.
>
> Nor is the positional data nearly as variable as some might suppose.
> A significant part of the positional error is 'bias" error, which
> doesn't change in the short term... ie it doesn't give the correct
> position but the incorrect position doesn't change... hence the speed
> readout isn't adversely affected.
>
> If you use a handheld GPS to measure an enclosed area, as I often do,
> the result will nearly always be identical to DGPS (for the same
> reason). Measure it twice with a handheld, and if the result doesn't
> change it will be correct... or at least the odds of it not being
> correct will be extremely large. The theoretical error, based on
> positional accuracy, is several percent... the practical error is more
> like 0.1%.
>

Stupid question (so I'm half expecting a stupid answer) but you do have
both units set to the same map datum I am assuming? In which the errors
should be pretty minimal in that case

Adam