The distance that my bike computer is measuring is not in agreement
with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

The alternative question is how does one get a reliable distance to
obtain a calibration.

I've just had an argument with myself as to whether I increase the
wheel calibration figure or reduce it to compensate for the 5% high
error I'm getting on the 4.0 miles, and have won the argument by
realising that the entered circumference is the multiplier for each
pulse from the sensor, so counter intuitively (initially to me anyway)
to reduce an over-mileage, you reduce the computer wheel calibration
figure.

Rob

robgraham wrote:
The distance that my bike computer is measuring is not in agreement
with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

The alternative question is how does one get a reliable distance to
obtain a calibration.

SatNav?

--
Adam

"ARWadsworth" wrote in message
...
robgraham wrote:
The distance that my bike computer is measuring is not in agreement
with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

The alternative question is how does one get a reliable distance to
obtain a calibration.


Ask a local policeman. They have markers at a set distance for calibrating
on board speed recorders.

Mike

"robgraham" wrote in message
...
The distance that my bike computer is measuring is not in agreement
with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

The alternative question is how does one get a reliable distance to
obtain a calibration.

I've just had an argument with myself as to whether I increase the
wheel calibration figure or reduce it to compensate for the 5% high
error I'm getting on the 4.0 miles, and have won the argument by
realising that the entered circumference is the multiplier for each
pulse from the sensor, so counter intuitively (initially to me anyway)
to reduce an over-mileage, you reduce the computer wheel calibration
figure.


Using Google Earth is not going to give accurate figures. Probably the way
to do it is to use a GPS and measure two marks on a length of road and then
check to odometer against it.


Peter Crosland

"robgraham" wrote in message
...
The distance that my bike computer is measuring is not in agreement
with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

The alternative question is how does one get a reliable distance to
obtain a calibration.

I've just had an argument with myself as to whether I increase the
wheel calibration figure or reduce it to compensate for the 5% high
error I'm getting on the 4.0 miles, and have won the argument by
realising that the entered circumference is the multiplier for each
pulse from the sensor, so counter intuitively (initially to me anyway)
to reduce an over-mileage, you reduce the computer wheel calibration
figure.

Rob

Heath Robinson method

Get a pram wheel with rigid rubber "tyre".
Measure diameter.
Multiply by pi for circumference.
Fix to handle and add revolution counter from bike shop.
Mark pavement.
Mark wheel and align with pavement mark.
Walk a fair distance along a defined route ensuring constant contact between
wheel and pavement. Stop at a full number of wheel rotations - mark
pavement.
Multiply number of rotations logged by circumference to give accurate
distance between points.
Travel between marks on bike and adjust your bike odometer to measure same.

Phil

On 30/09/2011 14:50, robgraham wrote:
The distance that my bike computer is measuring is not in agreement
with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

The alternative question is how does one get a reliable distance to
obtain a calibration.

I've just had an argument with myself as to whether I increase the
wheel calibration figure or reduce it to compensate for the 5% high
error I'm getting on the 4.0 miles, and have won the argument by
realising that the entered circumference is the multiplier for each
pulse from the sensor, so counter intuitively (initially to me anyway)
to reduce an over-mileage, you reduce the computer wheel calibration
figure.

Rob

THe way I do it is this:

In the cycle computer setup, there will be pre-sets for common wheel /
tyre sizes, like 700x23. Those are usually close.

But there's usually a 'other' option where you can manually enter a
number corresponding to the distance covered per revolution ( the wheel
circumference ). To get an accurate measure of this, make a mark on
the ground, set the wheel with the valve exactly down at the mark, and
with your normal weight on the bike, make exactly 10 revolutions and
make another mark. Measure the distance, and divide by 10.

THe reason I say to do it with your normal weight on is that you will
squidge the tyre to some extent, so best to do the calb under normal
load conditions.

--
Ron

On Fri, 30 Sep 2011 17:10:01 +0100, Ron Lowe wrote:

On 30/09/2011 14:50, robgraham wrote:
The distance that my bike computer is measuring is not in agreement
with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

The alternative question is how does one get a reliable distance to
obtain a calibration.

I've just had an argument with myself as to whether I increase the
wheel calibration figure or reduce it to compensate for the 5% high
error I'm getting on the 4.0 miles, and have won the argument by
realising that the entered circumference is the multiplier for each
pulse from the sensor, so counter intuitively (initially to me anyway)
to reduce an over-mileage, you reduce the computer wheel calibration
figure.

Rob

THe way I do it is this:

In the cycle computer setup, there will be pre-sets for common wheel /
tyre sizes, like 700x23. Those are usually close.

But there's usually a 'other' option where you can manually enter a
number corresponding to the distance covered per revolution ( the wheel
circumference ). To get an accurate measure of this, make a mark on
the ground, set the wheel with the valve exactly down at the mark, and
with your normal weight on the bike, make exactly 10 revolutions and
make another mark. Measure the distance, and divide by 10.

THe reason I say to do it with your normal weight on is that you will
squidge the tyre to some extent, so best to do the calb under normal
load conditions.

For my 700Cx28 tyres the 'book' says 214cm, carefully measuring 1 rev. gives
212.5cm, applying about 30kg on the bars gives 211.5cm so I use that.
The tyres are about 80psi; the other bike has 25mm tyres at 100psi and the
extra loading makes no discernable difference.

If you can find a measured mile[1] that far enough to eliminate most errors.
Bear in mind that 1 pass of the magnet adds 2m, even if it moves only 1cm
past the sensor.

[1] I've never managed to find a measured mile but did see in Leeds a sign
"End of Measured Mile". A chap in our group wondered aloud where the start
was - hilarious at first but sensible in that we hadn't seen it.
--
Peter.
The gods will stay away
whilst religions hold sway

On 01/10/2011 08:41, PeterC wrote:
[1] I've never managed to find a measured mile but did see in Leeds a sign
"End of Measured Mile". A chap in our group wondered aloud where the start
was - hilarious at first but sensible in that we hadn't seen it.

The Bristol-Bath Railway Path has markings at measured 100m intervals
along the length of the Avon Valley Railway (it also has a measured mile
between the Bitton and Riverside stations), which I used to calibrate my
computer.

It's worth bearing in mind that tyre pressure makes a small but
noticeable difference. My old commute used to be measured at 5.37 miles
at 100psi, 5.38 miles at 90psi and 5.39 miles at 80psi. I use my trip
distance for the ride to work to tell me when I need to to my tyres up.

--
Danny Colyer http://www.redpedals.co.uk
"I'm riding a unicycle with my pants down. This should be every boy's
dream." - Bartholomew J Simpson

On Sat, 1 Oct 2011 08:41:25 +0100, PeterC
wrote:

If you can find a measured mile[1] that far enough to eliminate most errors.
Bear in mind that 1 pass of the magnet adds 2m, even if it moves only 1cm
past the sensor.

[1] I've never managed to find a measured mile

The local traffic cops certainly used to know things like that - but
I'd be surprised if you could find one nowadays.

On Sat, 01 Oct 2011 10:36:30 +0100, Danny Colyer wrote:

On 01/10/2011 08:41, PeterC wrote:
[1] I've never managed to find a measured mile but did see in Leeds a sign
"End of Measured Mile". A chap in our group wondered aloud where the start
was - hilarious at first but sensible in that we hadn't seen it.

The Bristol-Bath Railway Path has markings at measured 100m intervals
along the length of the Avon Valley Railway (it also has a measured mile
between the Bitton and Riverside stations), which I used to calibrate my
computer.

That's interesting. I've ridden it a couple of times but just didn't notice
- it's also about 100 miles from here :-(

It's worth bearing in mind that tyre pressure makes a small but
noticeable difference. My old commute used to be measured at 5.37 miles
at 100psi, 5.38 miles at 90psi and 5.39 miles at 80psi. I use my trip
distance for the ride to work to tell me when I need to to my tyres up.

There's a pot hole[1] at a nominal mile from home and that gives me some
idea, although temperature also makes a difference.

[1] Usefully permanent nowadays.
--
Peter.
The gods will stay away
whilst religions hold sway

"PeterC" wrote in message
...

If you can find a measured mile[1] that far enough to eliminate most
errors.
Bear in mind that 1 pass of the magnet adds 2m, even if it moves only 1cm
past the sensor.

[1] I've never managed to find a measured mile but did see in Leeds a sign
"End of Measured Mile". A chap in our group wondered aloud where the start
was - hilarious at first but sensible in that we hadn't seen it.

There are measured markers on motorways, not much use for a bike though.
you could calibrate your car using them and then mark out a distance using
the car.

I just went for the easy option and got a bike computer that uses GPS and
doesn't care what size the wheels are.
Its also more useful as you can log tracks and put GPS coordinates into your
photos, etc.

On Oct 1, 8:27*pm, "dennis@home" wrote:
"PeterC" wrote in message

...

If you can find a measured mile[1] that far enough to eliminate most
errors.
Bear in mind that 1 pass of the magnet adds 2m, even if it moves only 1cm
past the sensor.

[1] I've never managed to find a measured mile but did see in Leeds a sign
"End of Measured Mile". A chap in our group wondered aloud where the start
was - hilarious at first but sensible in that we hadn't seen it.

There are measured markers on motorways, not much use for a bike though.
you could calibrate your car using them and then mark out a distance using
the car.

I just went for the easy option and got a bike computer that uses GPS and
doesn't care what size the wheels are.
Its also more useful as you can log tracks and put GPS coordinates into your
photos, etc.

Thanks guys - I'd forgotten that I've a got a hill walking gps that I
never use and not even sure how to use it; I'm a traditional map and
compass user. Certainly the gps system is the solution.
Rob

In article
..com, robgraham scribeth thus
On Oct 1, 8:27*pm, "dennis@home" wrote:
"PeterC" wrote in message

...

If you can find a measured mile[1] that far enough to eliminate most
errors.
Bear in mind that 1 pass of the magnet adds 2m, even if it moves only 1cm
past the sensor.

[1] I've never managed to find a measured mile but did see in Leeds a sign
"End of Measured Mile". A chap in our group wondered aloud where the start
was - hilarious at first but sensible in that we hadn't seen it.

There are measured markers on motorways, not much use for a bike though.
you could calibrate your car using them and then mark out a distance using
the car.

I just went for the easy option and got a bike computer that uses GPS and
doesn't care what size the wheels are.
Its also more useful as you can log tracks and put GPS coordinates into your
photos, etc.

Thanks guys - I'd forgotten that I've a got a hill walking gps that I
never use and not even sure how to use it; I'm a traditional map and
compass user. Certainly the gps system is the solution.
Rob

I should try Google earth. We checked this off a while ago against some
well known structures such as Airport runways and bits of rail line and
even the size of our own back yard and it was surprisingly accurate!...
--
Tony Sayer

PeterC wrote:


For my 700Cx28 tyres the 'book' says 214cm, carefully measuring 1
rev. gives 212.5cm, applying about 30kg on the bars gives 211.5cm so
I use that.

I'm confused by that, sorry. I can see that pressing down on the handlebars
reduces the radius (but significantly distorts the tyre from a circle).
However, the tyre must still lay down 212.5cms of rubber onto the road per
revolution. So, where does the extra 1cm go when you only get 211.5 with the
weight on? I'm assuming no slippage between the rubber and the floor.

On Sun, 2 Oct 2011 14:49:37 +0100, GB wrote:

PeterC wrote:


For my 700Cx28 tyres the 'book' says 214cm, carefully measuring 1
rev. gives 212.5cm, applying about 30kg on the bars gives 211.5cm so
I use that.

I'm confused by that, sorry. I can see that pressing down on the handlebars
reduces the radius (but significantly distorts the tyre from a circle).
However, the tyre must still lay down 212.5cms of rubber onto the road per
revolution. So, where does the extra 1cm go when you only get 211.5 with the
weight on? I'm assuming no slippage between the rubber and the floor.


No slippage as it's on the concrete path and the bearings are good. The
weight reduces the rolling radius, same a the tyre being under-pressure
does.
--
Peter.
The gods will stay away
whilst religions hold sway

In article , tony sayer
wrote:
[1] I've never managed to find a measured mile but
did see in Leeds a sign "End of Measured Mile". A
chap in our group wondered aloud where the start was
- hilarious at first but sensible in that we hadn't
seen it.
Could ask at the local cop shop. If they 'do' a driver by
following in a patrol (or other) vehicle, they then have to
check 'their' speedo over a measured mile using a certified
stopwatch



There are measured markers on motorways, not much use
for a bike though. you could calibrate your car using
them and then mark out a distance using the car.
The markers are placed in metric measurements. OK if your
speedo is set to kilometres, awkward if you're on miles per
hour

John

--
John Mulrooney
NOTE Email address IS correct but might not be checked for a while.

If you think you can - you can.

On Mon, 03 Oct 2011 15:29:33 +0100, JTM wrote:

In article , tony sayer
wrote:
[1] I've never managed to find a measured mile but
did see in Leeds a sign "End of Measured Mile". A
chap in our group wondered aloud where the start was
- hilarious at first but sensible in that we hadn't
seen it.
Could ask at the local cop shop. If they 'do' a driver by
following in a patrol (or other) vehicle, they then have to
check 'their' speedo over a measured mile using a certified
stopwatch

Good tip, thanks. ISTR a mention, many years ago, of a chained mile up the
A5 going SE from Towcester.


There are measured markers on motorways, not much use
for a bike though. you could calibrate your car using
them and then mark out a distance using the car.
The markers are placed in metric measurements. OK if your
speedo is set to kilometres, awkward if you're on miles per
hour

John

Simple conversion, innit.
--
Peter.
The gods will stay away
whilst religions hold sway

On 03/10/2011 15:29, JTM wrote:
The markers are placed in metric measurements. OK if your
speedo is set to kilometres, awkward if you're on miles per
hour

They're every 100 metres. Within my limits 1600 metres will do for a
mile. I don't mind being 9/1600 ~= 0.5% out.

Andy

On Mon, 03 Oct 2011 20:34:08 +0100, Andy Champ wrote:

On 03/10/2011 15:29, JTM wrote:
The markers are placed in metric measurements. OK if your
speedo is set to kilometres, awkward if you're on miles per
hour

They're every 100 metres. Within my limits 1600 metres will do for a
mile. I don't mind being 9/1600 ~= 0.5% out.

Andy

0.5% would have been about 130 miles out one year - that's a lot for
nothing!
The setting is to 1mm in ~2.14m; the measurement of the wheel is probably
about twice that, so 0.1%. Makes 0.5% look a bit coarse.
--
Peter.
The gods will stay away
whilst religions hold sway

"PeterC" wrote in message
...
On Mon, 03 Oct 2011 20:34:08 +0100, Andy Champ wrote:

On 03/10/2011 15:29, JTM wrote:
The markers are placed in metric measurements. OK if your
speedo is set to kilometres, awkward if you're on miles per
hour

They're every 100 metres. Within my limits 1600 metres will do for a
mile. I don't mind being 9/1600 ~= 0.5% out.

Andy

0.5% would have been about 130 miles out one year - that's a lot for
nothing!
The setting is to 1mm in ~2.14m; the measurement of the wheel is probably
about twice that, so 0.1%. Makes 0.5% look a bit coarse.

The weight distribution/pressure and other factors will change the rolling
radius by more than that so don't get the idea it actually matters.

On Tue, 4 Oct 2011 12:42:08 +0100, dennis@home wrote:

"PeterC" wrote in message
...
On Mon, 03 Oct 2011 20:34:08 +0100, Andy Champ wrote:

On 03/10/2011 15:29, JTM wrote:
The markers are placed in metric measurements. OK if your
speedo is set to kilometres, awkward if you're on miles per
hour

They're every 100 metres. Within my limits 1600 metres will do for a
mile. I don't mind being 9/1600 ~= 0.5% out.

Andy

0.5% would have been about 130 miles out one year - that's a lot for
nothing!
The setting is to 1mm in ~2.14m; the measurement of the wheel is probably
about twice that, so 0.1%. Makes 0.5% look a bit coarse.

The weight distribution/pressure and other factors will change the rolling
radius by more than that so don't get the idea it actually matters.

TFT! Had a front-tyre pucture on the way to Tescrot this morning. I'd
realised that the pressure was down a bit and by 5 miles measured it was
about 25 yards short of the usual place.
--
Peter.
The gods will stay away
whilst religions hold sway

On Oct 2, 2:49*pm, "GB" wrote:
PeterC wrote:

For my 700Cx28 tyres the 'book' says 214cm, carefully measuring 1
rev. gives 212.5cm, applying about 30kg on the bars gives 211.5cm so
I use that.

I'm confused by that, sorry. I can see that pressing down on the handlebars
reduces the radius (but significantly distorts the tyre from a circle).
However, the tyre must still lay down 212.5cms of rubber onto the road per
revolution. So, where does the extra 1cm go when you only get 211.5 with the
weight on? I'm assuming no slippage between the rubber and the floor.

Mildly interesting fact, road races (running, marathons etc) are
always measured to be longer than they need to account for the
inherent 0.1% error when using a calibrated bicycle measurer. So in
the worst case, a marathon would be 42 metres longer (could be 15-20
seconds for a world class marathon runner).

http://www.coursemeasurement.org.uk/director-1.htm

Matt

On Tue, 4 Oct 2011 07:47:19 -0700 (PDT), larkim wrote:

On Oct 2, 2:49*pm, "GB" wrote:
PeterC wrote:

For my 700Cx28 tyres the 'book' says 214cm, carefully measuring 1
rev. gives 212.5cm, applying about 30kg on the bars gives 211.5cm so
I use that.

I'm confused by that, sorry. I can see that pressing down on the handlebars
reduces the radius (but significantly distorts the tyre from a circle).
However, the tyre must still lay down 212.5cms of rubber onto the road per
revolution. So, where does the extra 1cm go when you only get 211.5 with the
weight on? I'm assuming no slippage between the rubber and the floor.

Mildly interesting fact, road races (running, marathons etc) are
always measured to be longer than they need to account for the
inherent 0.1% error when using a calibrated bicycle measurer. So in
the worst case, a marathon would be 42 metres longer (could be 15-20
seconds for a world class marathon runner).

http://www.coursemeasurement.org.uk/director-1.htm

Matt

That's changed! When I was riding time trials, mid-70s - mid-80s, our local
courses of up to 50 miles were measure with a 'fifth wheel' behind a car. It
was, I guess, the same as a road surveyor uses and the car would try to take
the same line as would a bike. This meant that the car would sometimes have
it's wheels just off the carriageway, so most measuring was done v. early on
a Sunday morning.
One local chap was measuring our club's 50 course on his bike with a
cyclometer as mentioned on the linked site. He'd done about 45 miles and had
a front-wheel puncture. Start again!
--
Peter.
The gods will stay away
whilst religions hold sway

On Fri, 30 Sep 2011 06:50:11 -0700 (PDT), robgraham
wrote:

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

You've got answers on the bike computer side...

GE will squish and stretch the photos to form one image. This will lead to
errors, some obvious like objects at the edges having two shadows.

I have found GPS coordinates of single points I've marked in the field to be
fairly close in North-South direction, and to be off by up to 50 meters or so in
East-West. Don't know if that's a local effect, or generally so.


Thomas Prufer

On Oct 5, 6:05*pm, Thomas Prufer prufer.pub...@mnet-
online.de.invalid wrote:
On Fri, 30 Sep 2011 06:50:11 -0700 (PDT), robgraham
wrote:

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

You've got answers on the bike computer side...

GE will squish and stretch the photos to form one image. This will lead to
errors, some obvious like objects at the edges having two shadows.

I have found GPS coordinates of single points I've marked in the field to be
fairly close in North-South direction, and to be off by up to 50 meters or so in
East-West. Don't know if that's a local effect, or generally so.

Thomas Prufer

Thanks Thomas - that's an interesting reply. The distance that I am
using is predominantly east-west so it will be interesting, once I
come to terms with getting my gps to work (!), to see if the distance
is correct as re-setting the calibration figure now gives a reasonable
distance agreement.

Rob

On Wed, 5 Oct 2011 14:16:40 -0700 (PDT), robgraham
wrote:

Thanks Thomas - that's an interesting reply. The distance that I am
using is predominantly east-west so it will be interesting, once I
come to terms with getting my gps to work (!), to see if the distance
is correct as re-setting the calibration figure now gives a reasonable
distance agreement.

The points are off, then they aren't I think the pictures are stretched
nonlinearly to join them at the seams, and presumably somehow matched up to
known lat/lon positions. So -- and I'll wave my hands a bit here -- the error
could be anything from a few tens of meters at each end, to a few tens of meters
at each point of the path, at the very worst...

I've usually done the "vertical valve stem to vertical valve stem" measurement.
Found it to be different enough from the value given in the table to be worth
doing.


Thomas Prufer

On Oct 5, 6:05*pm, Thomas Prufer prufer.pub...@mnet-
online.de.invalid wrote:
On Fri, 30 Sep 2011 06:50:11 -0700 (PDT), robgraham
wrote:

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

You've got answers on the bike computer side...

GE will squish and stretch the photos to form one image. This will lead to
errors, some obvious like objects at the edges having two shadows.

I have found GPS coordinates of single points I've marked in the field to be
fairly close in North-South direction, and to be off by up to 50 meters or so in
East-West. Don't know if that's a local effect, or generally so.

Thomas Prufer

Assuming GE uses the same overlays that Google Maps does, I've never
experienced anywhere near the deviations that you seem to be getting
when comparing the photos with the reality on the ground. To keep in
touch with my inner child, I'm a "geocacher", so using a GPSr to find
reasonably precise spots is a bit of a hobby of mine. Sometimes when
I don't have my GPSr with me I'll locate the place where something has
been hidden simply by looking at the aerial photo instead, and
invariably the location (e.g. a specific tree, stile, etc) is a 100%
match for the coordinates (to within a metre or so).

It could simply be that the maps / photos are much better aligned
around here than they are in your area. I was going to wonder whether
your GPSr is at fault, as obviously the distance between E/W points
varies dependent on your N/S, but I can't conceive of a GPSr not
taking this into account.

Matt