Garmin GPS - Odometer overestimates at walking speed

Author

Odometer overestimates at walking speed

Herbert Kanner

2007-09-16, 10:33 pm

In the course of calibrating a pedometer, I was checking it over a
straight-line walk. I measured the true distance at various times with
three different Garmin units: a 12, a 60C, and a 60Cx. Every time,
being at one point and asking for the distance to the other, I got .60
miles. Yesterday, I was repeating a check on a pedometer, but I also
first cleared the Garmin odometer. The latter claimed 6.4 miles walked,
even though it confirmed the distance as 6.0. The pedometer lived up to
expectations, coming up for the round trip with 12.2, well within the
error range of stride-length settings. And, I guarantee, I was not doing
S-curves as I walked.

For anyone in Palo Alto, CA who is interested, the two end points were
on Emerson Ave., from the near corner of Washington Ave. to the near
corner of Churchill Ave.

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Tom Stiller

2007-09-16, 10:33 pm

In article <kanner-28D91A.15235716092007@nnrp-virt.nntp.sonic.net>,
Herbert Kanner <kanner@deadspam.com> wrote:

> In the course of calibrating a pedometer, I was checking it over a
> straight-line walk. I measured the true distance at various times with
> three different Garmin units: a 12, a 60C, and a 60Cx. Every time,
> being at one point and asking for the distance to the other, I got .60
> miles. Yesterday, I was repeating a check on a pedometer, but I also
> first cleared the Garmin odometer. The latter claimed 6.4 miles walked,
> even though it confirmed the distance as 6.0. The pedometer lived up to
> expectations, coming up for the round trip with 12.2, well within the
> error range of stride-length settings. And, I guarantee, I was not doing
> S-curves as I walked.
>
> For anyone in Palo Alto, CA who is interested, the two end points were
> on Emerson Ave., from the near corner of Washington Ave. to the near
> corner of Churchill Ave.

I have had the GPS pick up a stray point while walking. When this
happens, both the average speed and odometer are affected.

--
Tom Stiller

PGP fingerprint = 5108 DDB2 9761 EDE5 E7E3 7BDA 71ED 6496 99C0 C7CF

Hinkle

2007-09-16, 10:33 pm

If you can, check out my post on 8/11/07 re a problem with my 76CSx. You
might be having a similar problem. My unit worked fine but then, all of a
sudden, it started behaving badly.

I phoned Garmin and we went through some reset/troubleshooting and then they
had me send it in for service. Only took a few days until a new one
arrived UPS.

Ralph

Simon Slavin

2007-09-18, 10:33 pm

On 16/09/2007, Herbert Kanner wrote in message <kanner-
28D91A.15235716092007@nnrp-virt.nntp.sonic.net>:

> Yesterday, I was repeating a check on a pedometer, but I also
> first cleared the Garmin odometer. The latter claimed 6.4 miles walked,
> even though it confirmed the distance as 6.0. The pedometer lived up to
> expectations, coming up for the round trip with 12.2, well within the
> error range of stride-length settings. And, I guarantee, I was not doing
> S-curves as I walked.

A pedometer measures the (handwave) number of paces you take. It's rather
precise once, as you point out, it has been calibrated for your pace-
length.

A GPS receiver measures distance by taking fixes at intervals, and
supplying the total of the distances between successive points. The
problem is that each fix is going to be slightly inaccurate, because a GPS
receiver is slightly inaccurate.

Suppose you were travelling exactly west to east and, quite by accident
fixes 6 and 8 were completely accurate. If fix 7 gives your position as
too far north then the total distance from fix 6 to fix 8 will be too far,
even though the receiver does agree that you got back on track again.
Now, your GPS receiver has a computer in and could do some post-processing
to smooth out the fixes it takes in working out path length. The problem
is that this may actually give a less accurate result than the original
one, especially if the path you're taking really is a jagged line, which
is common in people moving over difficult terrain. The smoothing
algorithm will round away all the corners.

So yes, GPS receivers routinely overestimate total distances travelled.
The more accurate the fixes they get, the smaller the error will be.

Simon.
--
http://www.hearsay.demon.co.uk

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