Accuracy of Fitsense

Bert Duplessis said:

I wore a Fitsense watch & foot pod last Sunday in the Houston Marathon; it
displayed a total distance of 26.1 miles which is not bad. Initially I
struggled with getting reliable mileage data on the FS, but eventually figured
out that the 'secret' is to strap the foot pod in EXACTLY the same spot on the
same type of shoes every time. Just an inch higher or lower on the foot can
make a major difference in the mileage reading. Of course it is also
important to get the calibration right.

Even then, it's not foolproof -- try running on a loop course 2 miles long
or more, and see how consistent it is. Mine would vary by as much as around
1-2%. For accurate distance measurement, go low-tech -- nothing beats a
bicycle (there's a reason why they use bicycles to measure road race courses)

Cheers,
--
Donovan Rebbechi
http://pegasus.rutgers.edu/~elflord/

Quoted message said:

Even then, it's not foolproof -- try running on a loop course 2 miles long
or more, and see how consistent it is. Mine would vary by as much as around
1-2%. For accurate distance measurement, go low-tech -- nothing beats a
bicycle (there's a reason why they use bicycles to measure road race courses)

Gotta be careful even with a bicycle; my odometer is off by as much as
7% sometimes. Calibration is still key -- as I understand it the
people measuring race courses adjust their odometers for expected
wobble, and they're pretty careful about tire pressure, which can
really throw a wrench into things.

I've had good luck with Fitsense since I took the precaution of
calibrating it separately for each different kind of shoe I wear.

--
Brian P. Baresch
Fort Worth, Texas, USA
Professional editing and proofreading

If you're going through hell, keep going. --Winston Churchill

Brian Baresch said:

Quoted message said:

Even then, it's not foolproof -- try running on a loop course 2 miles long
or more, and see how consistent it is. Mine would vary by as much as around
1-2%. For accurate distance measurement, go low-tech -- nothing beats a
bicycle (there's a reason why they use bicycles to measure road race courses)

Gotta be careful even with a bicycle; my odometer is off by as much as
7% sometimes. Calibration is still key -- as I understand it the
people measuring race courses adjust their odometers for expected
wobble, and they're pretty careful about tire pressure, which can
really throw a wrench into things.

Yes, I posted at length about this, for the benefit of anyone who cares.

They do multiple rides of a straight calibration course about 400m long just
before and just after measuring the course so tire pressure isn't that much of
an issue because calibration is fresh and doing it both before and after
keeps means that if tire pressure changes, you can still bound the error.

Having said that, I'm surprised that a bike odometer would be off by 7%, unless
it was a systematic error -- I don't see how the range in circumference could
be more than a couple of cm out of about 200cm circumference, so you're
talking a percent or so (for example, a half cm difference in radius is a 3cm
difference in circumference, which would account for about 1.5%). Unless it's
just misconfigured.

Cheers,
--
Donovan Rebbechi
http://pegasus.rutgers.edu/~elflord/

Quoted message said:

They do multiple rides of a straight calibration course about 400m long just
before and just after measuring the course so tire pressure isn't that much of
an issue because calibration is fresh and doing it both before and after
keeps means that if tire pressure changes, you can still bound the error.

Would you still have to be careful about checking the tire pressure
each time you went to measure a course?

Quoted message said:

Having said that, I'm surprised that a bike odometer would be off by 7%, unless
it was a systematic error -- I don't see how the range in circumference could
be more than a couple of cm out of about 200cm circumference, so you're
talking a percent or so (for example, a half cm difference in radius is a 3cm
difference in circumference, which would account for about 1.5%). Unless it's
just misconfigured.

Misconfiguration is possible. I now have a fairly cheap (Specialized,
bottom of the line) odometer that doesn't display hundredths of a
mile; my previous one would sometimes read as much as 1.07 for what I
was pretty sure was a mile. Wobble and occasional low air pressure
would account for some of it, I'm sure.

--
Brian P. Baresch
Fort Worth, Texas, USA
Professional editing and proofreading

If you're going through hell, keep going. --Winston Churchill

Brian Baresch said:

Quoted message said:

They do multiple rides of a straight calibration course about 400m long just
before and just after measuring the course so tire pressure isn't that much
of an issue because calibration is fresh and doing it both before and after
keeps means that if tire pressure changes, you can still bound the error.

Would you still have to be careful about checking the tire pressure
each time you went to measure a course?

I've seen examples where people measure it, but I don't quite understand why,
as the calibration should take care of it, since you calibrate each time you
measure the course (you really do need to calibrate each time because tire
pressure isn't the only variable). I suppose it's probably a good idea to
pump up the tires fairly hard. Maybe they record it as a way of making it
clear they pumped up their tires properly.

Quoted message said:

Misconfiguration is possible. I now have a fairly cheap (Specialized,
bottom of the line) odometer that doesn't display hundredths of a
mile;

Speaking of "hundredths of a mile", the way to make the bike odometer
method *really* accurate is to set the wheel circumference to 250cm, put
the display in metric, and then put 4 magnets on the wheel -- then the
display counts wheel revolutions (because the display will increase by
4x250cm = 0.01km for each wheel revolution). Then if you set the start point
carefully, you can count off spokes at the end of the ride, to obtain a
fractional count (which you need to accurately measure the calibration
course).

Cheers,
--
Donovan Rebbechi
http://pegasus.rutgers.edu/~elflord/

"Donovan Rebbechi" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

On 2005-01-21, Brian Baresch <[email hidden]>

Quoted message said:

Quoted message said:

They do multiple rides of a straight calibration course about 400m long
just
before and just after measuring the course so tire pressure isn't that
much
of an issue because calibration is fresh and doing it both before and
after
keeps means that if tire pressure changes, you can still bound the error.

Would you still have to be careful about checking the tire pressure
each time you went to measure a course?

I've seen examples where people measure it, but I don't quite understand
why,
as the calibration should take care of it, since you calibrate each time
you
measure the course (you really do need to calibrate each time because tire
pressure isn't the only variable). I suppose it's probably a good idea to
pump up the tires fairly hard. Maybe they record it as a way of making it
clear they pumped up their tires properly.

Quoted message said:

Misconfiguration is possible. I now have a fairly cheap (Specialized,
bottom of the line) odometer that doesn't display hundredths of a
mile;

Speaking of "hundredths of a mile", the way to make the bike odometer
method *really* accurate is to set the wheel circumference to 250cm, put
the display in metric, and then put 4 magnets on the wheel -- then the
display counts wheel revolutions (because the display will increase by
4x250cm = 0.01km for each wheel revolution). Then if you set the start
point
carefully, you can count off spokes at the end of the ride, to obtain a
fractional count (which you need to accurately measure the calibration
course).

As a RD who measures courses, I'll say that a good way to avoid
the tire pressure issue is to use a professional big surveyors wheel
with a solid rubber tire. I have a really good one for my company.

What we do is periodically make sure it's in working order by
measuring out 100' (about 30m) on a flat surface - this is done with
steel measuring tape held to a tension of 5lbs, using a fish-hook scale.
Then the wheel is walked along it several times, each direction. Thus
the wheel is 'certified' to an acceptable level in order to measure
courses. When that is done, the shortest possible route is walked.
We go in both directions three times each (two different people,
independently).

This year we are getting all our routes certified by the ORA to meet
other standards, but they'll find them to be very accurate, I believe.

cheers,
--
David (in Hamilton, ON)
www.allfalldown.org
www.absolutelyaccurate.com

SwStudio said:

As a RD who measures courses, I'll say that a good way to avoid
the tire pressure issue is to use a professional big surveyors wheel
with a solid rubber tire.

There's no tire pressure issue so long as the tires are pumped up. There's a
calibration issue either way.

Your measurement proceedure is similar to that used by RRTC, which is
documented here:

http://www.rrtc.net/book/

They require a much longer calibration course though -- the minimum is 10
lengths of the 100' tape.

Cheers,
--
Donovan Rebbechi
http://pegasus.rutgers.edu/~elflord/

"Donovan Rebbechi" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

SwStudio said:

As a RD who measures courses, I'll say that a good way to avoid
the tire pressure issue is to use a professional big surveyors wheel
with a solid rubber tire.

There's no tire pressure issue so long as the tires are pumped up. There's
a
calibration issue either way.

I guess what I mean to say was that because there's no air
in the surveyors wheel tire, there's less variable in terms of
minute circumference differences. Sure, you make sure the
tires are pumped up but I can skip that step.

Quoted message said:

Your measurement proceedure is similar to that used by RRTC, which is
documented here:

http://www.rrtc.net/book/

They require a much longer calibration course though -- the minimum is 10
lengths of the 100' tape.

Right - I've seen that and it's a great site for that sort of stuff.

cheers,
--
David (in Hamilton, ON)
www.allfalldown.org
www.absolutelyaccurate.com

Quoted message said:

I've seen examples where people measure it, but I don't quite understand why,
as the calibration should take care of it, since you calibrate each time you
measure the course (you really do need to calibrate each time because tire
pressure isn't the only variable).

OK, now it's getting through my thick skull; don't know why I was so
slow about comprehending. Thanks for the clarification.

--
Brian P. Baresch
Fort Worth, Texas, USA
Professional editing and proofreading

If you're going through hell, keep going. --Winston Churchill