Re: Slightly off target?

On 12/14/05, mfkenney@gmail.com <mfkenney@gmail.com> wrote:

Nick,

Nick B wrote:
> Obviously I cannot be absolutely sure the GPS data is itself off the mark,
> or my code, but quite a lot of it is not correct. In fact, I must have swum
> across the canal a few times to get to the other side!
>
> Anyone else noticed this, or is this something to expect?

It looks like your path is ~20 meters off which is a bit high but it is
hard to tell where the problem lies without more information.  Having a
"3D fix" does not tell the whole story. Does your GPS provide an error
estimate?

I'm not sure it does, certainly not seen one. It's a Garmin GPS60. I thought the  WAAS part was a radio signal to notify my GPS of timing errors from the available satellites, hence reducing inaccuracies to hardly anything.

Most handheld GPS units process the location updates with a Kalman
filter.  This is nice as the unit will continue to estimate your
location even if it loses satellite reception for short periods of
time, e.g. passing through a grove of trees or through a short tunnel.
The flip side of this is a delayed response when the satellites are
back in view causing the error in your track to persist.

Yup, I've seen it continuing to estimate when I'm in tunnels. 

Next time you ride this route, keep an eye on the GPS error estimate
(and the other eye on the road of course ;)

I've ridden other routes and it's spot on, so I guess it must be my GPS estimating or something.

Thanks for the advice.