GPS distance measuring anomaly?

Hello;

I know just a bit more than nothing about GPS units. Despite
that, my brother who lives on the other side of Canada from me
(in BC) phoned to ask me how I would explain an anomaly in the
distance a GPS gives versus the distance a car odometer gives
over the same route. ((?))

The facts as I know them are as follows:

1) He has a regular recreational pedal biking route.
2) Three different motor vehicles have given distances ranging
from 35.5 to 35.7 kilometers over that route
3) When he takes his bicycle his GPS unit gives a distance of
33.3 kilometers.
4) The GPS unit also tells him that his accumulated rise during
the trip is about 1000 meters. That is a bunch of up and
down hills (not that the highest point is 1000 meters above
the lowest point). I doubt if this is relevant but he
mentioned it.


Dumb questions I did not think to ask:

- What was the make and model of the GPS unit.
- Has he driven the route with the GPS in the car - I think
not.

- I did ask about bald tires, and whether he was driving a
Honda, which sometimes gives a distance that is greater than
real. No dice on either of those.


So my question is:

Is there something about GPS's that would naturally explain
this effect?

or

Something else that ordinary people regularly ignore that
would explain it?


Thanks, if you read this far;

Friar Broccoli
Robert Keith Elias, Quebec, Canada Email: EliasRK (of) gmail * com
Best programmer's & all purpose text editor: http://www.semware.com

--------- I consider ALL arguments in support of my views ---------

For the moment we will assume the odometer is perfect!
And for the moment we will assume that the GPS is fixing its position
perfectly at each sample!
A GPS measures by straight line segments between sample points along
the route!
In our assumed perfect world, the GPS and odometer would agree on a
perfectly straight road!
However, when we introduce a curved route, the GPS measures straight
line shortcuts across the turns, which will be a function of the rate
of the curve and the sample rate! Under these conditions, the GPS
will show a shorter trip! You can increase the sample rate to get a
better fit, but you will use more memory and thus reduce the length of
a route that you can store! If you download the GPS trip to a map,
the effect is easily seen on winding roads!
Now, if you like, you can introduce other possible errors back into
the equation,, ha!!
Hope this helps,
SKM

wrote:




Yes - the fact that all car odometers read 'high'. It's all to do
with car makers not wanting to be sued by motorists going
'accidentally' over the speed limit. Any make/model of odometer will
theoretically vary by a few percent from 'dead accurate' (all sorts of
reasons including different tyre makes & profiles). So they build in
a 10% over-estimate which means that even with all possible variations
the actual speed will NEVER be more than the indicated speed.

Try the GPS in the car - you will see at once that the GPS speed is
lower than the indicated speedometer/odometer speed. The GPS will
still have an error, but not as great as the odometer error, and not
biased to be higher than actual.

Example - at 100kph true, the odometer will indicate from 110 - 115,
the GPS from 99 - 101.

Jim H

On Sun, 11 Mar 2007 10:59:21 +0000, Jim Hutton wrote:


Actually, in my 1999 Dodge (truck), the opposite is true. At 55 mph
indicated on the truck's speedometer, my GPS (a GPSMap 76S) shows about
57.5 mph. So, my truck under-registers. Our 2006 Honda Odyssey minivan
is just about dead on the money, though.
--

---- Len Philpot -------- l e n @ p h i l p o t . o r g (no spaces)

My experience is similar; my Toyota reads about 4% low in speed and milage,
and I believe my previous car was the similar.

I remember hearing the "rule" that speedometers were required to read high
when I first started driving, and that was my experience at that time, but
it doesn't seem to apply today, at least not in the USA; maybe it is
required by law in some countries.

When I first got my Jeep a friend had just gotten a Jimmy, and one weekend
we were driving around in the mountains above the Pacific Coast Highway.
Everything was going great, until we got on the main highway and he took off
like a bat out of hell! For a while I tried staying with him, till I
noticed a CHP on an adjacent side road. He later discovered that they had
put non-standard wheels on his Jimmy when he bought it, but had not changed
the speedometer gear ratio to match the new tires.

John