Bicycle Computer Altimeter Accuracy

[Colin Campbell]

Yesterday, I did one of Southern California's classic mountain rides -
Glendora Mountain Road. According to my bike computer, I did 1298
meters of climbing, and 1541 meters of descending (ending up exactly
where I started).

Is there any way of guessing which of these numbers might be more accurate?

 

[Ron Hardin]

Colin Campbell wrote:
>
> Yesterday, I did one of Southern California's classic mountain rides -
> Glendora Mountain Road. According to my bike computer, I did 1298
> meters of climbing, and 1541 meters of descending (ending up exactly
> where I started).
>
> Is there any way of guessing which of these numbers might be more accurate?

There's actually no correct number, for the answer depends on how much rise or fall
actually counts.

If you're riding on a random surface with wheel diameter D, the total vertical
excursion rises like 1 / sqrt(D), going to infinity as the diameter goes to zero.

The real world would just notice that things count that you don't want to count,
as the height wobbles get smaller ; yet they contribute the most to the total.

You'd like the up and down totals to agree, though.
--
[email protected]

On the internet, nobody knows you're a jerk.

 

[Ted Bennett]

Colin Campbell <[email protected]> wrote:

> Yesterday, I did one of Southern California's classic mountain rides -
> Glendora Mountain Road. According to my bike computer, I did 1298
> meters of climbing, and 1541 meters of descending (ending up exactly
> where I started).
>
> Is there any way of guessing which of these numbers might be more accurate?

Your starting point has apparently dropped by 243 meters. Happens in
Southern California all the time­it's those pesky subduction zones.

In my experience those altimeters are fairly useless. Since they figure
altitude by measuring air pressure fluctuations, which changes according
to temperature as well as altitude, there is built-in inaccuracy. Add
to that the cheapness of the instrument and you're bound to get the sort
of "measurements" you did.

If you want more accuracy, get a GPS which measures your altitude by
triangulation from satellites rather than air pressure. Usually within
one meter.

--
Ted Bennett

 

[Clive George]

"Ted Bennett" <[email protected]> wrote in message
news:tedbennett-F2FCF5.12520517092007@earthlink.vsrv-sjc.supernews.net...

> In my experience those altimeters are fairly useless. Since they figure
> altitude by measuring air pressure fluctuations, which changes according
> to temperature as well as altitude, there is built-in inaccuracy.

They do measure temperature as well - well, the Ciclosport ones do anyway. I
agree one which didn't wouldn't be very useful.

> Add
> to that the cheapness of the instrument and you're bound to get the sort
> of "measurements" you did.
>
> If you want more accuracy, get a GPS which measures your altitude by
> triangulation from satellites rather than air pressure. Usually within
> one meter.

That would be why the GPSs which record height have a barometric altimeter
on them as well?

cheers,
clive

 

[Lou Holtman]

Colin Campbell wrote:
> Yesterday, I did one of Southern California's classic mountain rides -
> Glendora Mountain Road. According to my bike computer, I did 1298
> meters of climbing, and 1541 meters of descending (ending up exactly
> where I started).
>
> Is there any way of guessing which of these numbers might be more accurate?

(1541 + 1298)/2 = 1420 m, is your best bet.

Lou
--
Posted by news://news.nb.nu (http://www.nb.nu)

 

[Ted Bennett]

"Clive George" wrote:

> "Ted Bennett" wrote

> > If you want more accuracy, get a GPS which measures your altitude by
> > triangulation from satellites rather than air pressure. Usually within
> > one meter.
>
> That would be why the GPSs which record height have a barometric altimeter
> on them as well?

Some do it by barometric pressure measurement, actually most do.
However units used in aviation use the satellites for better accuracy.
Hardly practical for bicycle use, I would agree.

--
Ted Bennett

 

[David L. Johnson]

Clive George wrote:
> "Ted Bennett" <[email protected]> wrote in message

>> If you want more accuracy, get a GPS which measures your altitude by
>> triangulation from satellites rather than air pressure. Usually within
>> one meter.
>
> That would be why the GPSs which record height have a barometric
> altimeter on them as well?

A GPS uses the barometer primarily, with the satellites used as a
periodic check, since the satellite data is not all that accurate for
altitude, but the barometer is subject to drift due to atmospheric
conditions.

--

David L. Johnson

Some people used to claim that, if enough monkeys sat in front of
enough typewriters and typed long enough, eventually one of them would
reproduce the collected works of Shakespeare. The internet has
proven this not to be the case.

 

[Clive George]

"Ted Bennett" <[email protected]> wrote in message
news:tedbennett-429A10.13261717092007@earthlink.vsrv-sjc.supernews.net...

>> That would be why the GPSs which record height have a barometric
>> altimeter
>> on them as well?
>
> Some do it by barometric pressure measurement, actually most do.
> However units used in aviation use the satellites for better accuracy.
> Hardly practical for bicycle use, I would agree.

T'other way round - the barometric is more accurate. At a guess, a plane
doesn't really care that much about a 10m altitude difference, until it gets
close enough to the ground that the pilot can work that bit out. (fx: reads
Garmin website. It appears they don't use GPS for altitude unless the main
barometric one has died).

cheers,
clive

 

[Ted Bennett]

"David L. Johnson" <[email protected]> wrote:

> Clive George wrote:
> > "Ted Bennett" <[email protected]> wrote in message
>
> >> If you want more accuracy, get a GPS which measures your altitude by
> >> triangulation from satellites rather than air pressure. Usually within
> >> one meter.
> >
> > That would be why the GPSs which record height have a barometric
> > altimeter on them as well?
>
> A GPS uses the barometer primarily, with the satellites used as a
> periodic check, since the satellite data is not all that accurate for
> altitude, but the barometer is subject to drift due to atmospheric
> conditions.

Hmm. That differs from my understanding, which results from my
conversations with pilots. But I'm willing to be corrected, as pilots
are likely not to be GPS experts. Can anyone else who is knowledgeable
chime in?

--
Ted Bennett

 

[Ted Bennett]

"Clive George" <[email protected]> wrote:

> "Ted Bennett" <[email protected]> wrote
>
> >> That would be why the GPSs which record height have a barometric
> >> altimeter on them as well?
> >
> > Some do it by barometric pressure measurement, actually most do.
> > However units used in aviation use the satellites for better accuracy.
> > Hardly practical for bicycle use, I would agree.
>
> T'other way round - the barometric is more accurate. At a guess, a plane
> doesn't really care that much about a 10m altitude difference, until it gets
> close enough to the ground that the pilot can work that bit out. (fx: reads
> Garmin website. It appears they don't use GPS for altitude unless the main
> barometric one has died).
>
> cheers,
> clive

This may be nearly unheard of on this group: I was wrong.

Shows where repeating hearsay can lead one astray. Thanks.

Ted

--
Ted Bennett

 

[[email protected]]

On Sep 17, 12:52 pm, Ted Bennett <[email protected]> wrote:
> Colin Campbell <[email protected]> wrote:
> > Yesterday, I did one of Southern California's classic mountain rides -
> > Glendora Mountain Road. According to my bike computer, I did 1298
> > meters of climbing, and 1541 meters of descending (ending up exactly
> > where I started).
>
> > Is there any way of guessing which of these numbers might be more accurate?
>
> Your starting point has apparently dropped by 243 meters. Happens in
> Southern California all the time­it's those pesky subduction zones.
>
> In my experience those altimeters are fairly useless. Since they figure
> altitude by measuring air pressure fluctuations, which changes according
> to temperature as well as altitude, there is built-in inaccuracy. Add
> to that the cheapness of the instrument and you're bound to get the sort
> of "measurements" you did.
>

I don't have such a computer, but I have a watch with a barometric
altimeter, which is very likely similar. It uses some kind of lookup
table that relies on an average run of temperature and pressure
with altitude (possibly the International standard atmosphere).
In general it is quite accurate in a _differential_ sense
but the zeropoint will be off. For example, when I hike up an
Arizona mountain in summer, it's systematically off because the
temperature at 10,000 feet is higher than the watch is expecting.
However, the differential between bottom and top is fairly
stable as long as the weather doesn't change.

The other effect is the weather. It's a barometer, so of course
the readings at a fixed point will be different if a front blows
thrpugh
and the pressure changes.

In the OP's case, the readings might be different if e.g. the weather
changed, or he started off very early in the morning when it was
cold at the base, or something like that. It's easy to check a
single climb against a topo map, but harder if the computer's
number is "total climbing" from cumulating all the small ups and
downs.

Ben

 

[David L. Johnson]

Ted Bennett wrote:
> "David L. Johnson" <[email protected]> wrote:

>> A GPS uses the barometer primarily, with the satellites used as a
>> periodic check, since the satellite data is not all that accurate for
>> altitude, but the barometer is subject to drift due to atmospheric
>> conditions.
>
> Hmm. That differs from my understanding, which results from my
> conversations with pilots. But I'm willing to be corrected, as pilots
> are likely not to be GPS experts. Can anyone else who is knowledgeable
> chime in?
>
I got this from Garmin, IIRC.

--

David L. Johnson

"What am I on? I'm on my bike, six hours a day, busting my ass.
What are you on?"
--Lance Armstrong

 

[Colin Campbell]

Lou Holtman wrote:
> Colin Campbell wrote:
>> Yesterday, I did one of Southern California's classic mountain rides -
>> Glendora Mountain Road. According to my bike computer, I did 1298
>> meters of climbing, and 1541 meters of descending (ending up exactly
>> where I started).
>>
>> Is there any way of guessing which of these numbers might be more
>> accurate?
>
> (1541 + 1298)/2 = 1420 m, is your best bet.
>
> Lou
Yes, I'll take it. I'll tell the metrically challenged (nearly
everybody!) that I did about 4,500 feet.

 

[Clive George]

"Colin Campbell" <[email protected]> wrote in message
news:[email protected]...

> Yes, I'll take it. I'll tell the metrically challenged (nearly
> everybody!) that I did about 4,500 feet.

Ah, the joy of mixing one's units. I prefer distance in miles but height in
metres - and annoyingly my ciclosport won't let me choose that :-(

cheers,
clive

 

[!Jones]

On Mon, 17 Sep 2007 12:11:33 -0700, in rec.bicycles.tech Colin
Campbell <[email protected]> wrote:

>Yesterday, I did one of Southern California's classic mountain rides -
>Glendora Mountain Road. According to my bike computer, I did 1298
>meters of climbing, and 1541 meters of descending (ending up exactly
>where I started).
>
>Is there any way of guessing which of these numbers might be more accurate?

Yeah, the issue is that your front wheel is probably a 700 and your
rear is 27". Take the ratio and that's what you *would* have done.

Jones

 

[!Jones]

On Mon, 17 Sep 2007 15:34:42 -0400, in rec.bicycles.tech Ron Hardin
<[email protected]> wrote:

>You'd like the up and down totals to agree, though.

Personally, I prefer the downhill... I'm not into deferred
gratification.

Jones

 

[KF]

On Sep 17, 3:01 pm, "Clive George" <[email protected]> wrote:
> T'other way round - the barometric is more accurate. At a guess, a plane
> doesn't really care that much about a 10m altitude difference, until it gets
> close enough to the ground that the pilot can work that bit out. (fx: reads
> Garmin website. It appears they don't use GPS for altitude unless the main
> barometric one has died).

Not necessarily so, but let's at least define what we mean by
"accurate."
Take a GPS altimeter reading. With a decent "view" of the sky, it's
almost always within 20 feet or so of the correct altitude and NEVER
varies as the barometer fluctuates. So if you don't know where you are
and don't know what the barometric trend has been, the GPS
gives far more reliable elevation information.

On the other hand if you have just calibrated your barometric
altimeter and you climb a small hill (say, 200 feet high) the
barometric altimeter wil be able to tell you quite accurately how high
the top of the hill is, and even without calibration, can tell you how
much you have gained (the difference in elevation), probably to within
a few feet (although not all have that kind of fine resolution). But
try to measure the top of the hill again tomorrow without
recalibrating and you could easily be off by a couple hundred feet
just due to normal atmospheric changes. The GPS will always be
subject to the +/- 20 feet at any one location but will very rarely be
off more than that. So while the barometric altimeter under ideal
conditions may tell you that the hill is 200 +/- 5 feet high, under
unknown barometric changes it may really only tell you that the hill
is 200 +/- 200 feet high.
And if we change the hill to 2000 feet and still use the same +/-
figures, suddenly the +/-20 feet of the GPS starts to look VERY
accurate.

My experience from a number of years of use of both types of
instruments is that I'll take the overall consistency/accuracy of the
GPS over the barometric altimeter any day, in fact the GPS is about
the only way to conveniently reset a baromeric altimeter after unknown
changes in elevation/barometric pressure.

DR

 

[mitosis]

Colin Campbell <[email protected]> wrote:

> Yesterday, I did one of Southern California's classic mountain rides -
> Glendora Mountain Road. According to my bike computer, I did 1298
> meters of climbing, and 1541 meters of descending (ending up exactly
> where I started).
>
> Is there any way of guessing which of these numbers might be more accurate?

Your starting point has apparently dropped by 243 meters. Happens in
Southern California all the time*it's those pesky subduction zones.

In my experience those altimeters are fairly useless. Since they figure
altitude by measuring air pressure fluctuations, which changes according
to temperature as well as altitude, there is built-in inaccuracy. Add
to that the cheapness of the instrument and you're bound to get the sort
of "measurements" you did.

If you want more accuracy, get a GPS which measures your altitude by
triangulation from satellites rather than air pressure. Usually within
one meter.

--
Ted Bennett

I think you will find altitude by air pressure is more accurate than by GPS. The better GPS devices that also measure altitude defer to air pressure changes rather than satellite triangulation. Aircraft use air pressure changes for their altimeters, correcting them with pressure readings collected by radio enroute.

 

[mitosis]

On Sep 17, 3:01 pm, "Clive George" <[email protected]> wrote:
> T'other way round - the barometric is more accurate. At a guess, a plane
> doesn't really care that much about a 10m altitude difference, until it gets
> close enough to the ground that the pilot can work that bit out. (fx: reads
> Garmin website. It appears they don't use GPS for altitude unless the main
> barometric one has died).

Not necessarily so, but let's at least define what we mean by
"accurate."
Take a GPS altimeter reading. With a decent "view" of the sky, it's
almost always within 20 feet or so of the correct altitude and NEVER
varies as the barometer fluctuates. So if you don't know where you are
and don't know what the barometric trend has been, the GPS
gives far more reliable elevation information.

On the other hand if you have just calibrated your barometric
altimeter and you climb a small hill (say, 200 feet high) the
barometric altimeter wil be able to tell you quite accurately how high
the top of the hill is, and even without calibration, can tell you how
much you have gained (the difference in elevation), probably to within
a few feet (although not all have that kind of fine resolution). But
try to measure the top of the hill again tomorrow without
recalibrating and you could easily be off by a couple hundred feet
just due to normal atmospheric changes. The GPS will always be
subject to the +/- 20 feet at any one location but will very rarely be
off more than that. So while the barometric altimeter under ideal
conditions may tell you that the hill is 200 +/- 5 feet high, under
unknown barometric changes it may really only tell you that the hill
is 200 +/- 200 feet high.
And if we change the hill to 2000 feet and still use the same +/-
figures, suddenly the +/-20 feet of the GPS starts to look VERY
accurate.

My experience from a number of years of use of both types of
instruments is that I'll take the overall consistency/accuracy of the
GPS over the barometric altimeter any day, in fact the GPS is about
the only way to conveniently reset a baromeric altimeter after unknown
changes in elevation/barometric pressure.

DR

Barometric altimeters in aircraft are set according to local pressures.

 

[KF]

On Sep 17, 9:25 pm, mitosis <mitosis.2x3...@no-
mx.forums.cyclingforums.com> wrote:
> KF Wrote:
> > .... in fact the GPS is about
> > the only way to conveniently reset a baromeric altimeter after unknown
> > changes in elevation/barometric pressure.
>
> > DR
>
> Barometric altimeters in aircraft are set according to local
> pressures.
>

Yes, and typically at *known* field elevations, but not "after unknown
changes in elevation/barometric pressure"

DR