[OSM-talk] Conflating Tracks?

[Karl Newman]

On Tue, Mar 10, 2009 at 6:44 AM, Ed Avis <eda at waniasset.com> wrote:

> Daniel A Carleton <daniel.carleton <at> exaptic.com> writes:
>
> >Are any of you aware of a conflation algorithm that "snaps" GPS tracks
> >to the street grid?  This could be a useful way to verify existing
> >road networks and fill in any gaps.  e.g. I go out and ride on city
> >streets, overlay my GPS track on the OSM grid, and the algorithm shows
> >me where I traveled over unknown segments of road or deviated a
> >disproportionate amount.
>
> To do that you'd ideally want to know the margin of error for the GPS
> readings.
>  My GPS unit (Garmin eTrex Vista HCx) shows an error circle on its screen
> (giving a tighter circle when there is good reception, and a wider one when
> the
> sky is obscured by tall buildings) but on saving the tracks as GPX this
> information is lost.  Is there a way to export the track log including
> error
> circles into a format that the OSM servers recognize?
>
> (It appears that I could connect the unit to a laptop and record the tracks
> with
> gpsd, but I would prefer to just carry the unit and recover the track log
> afterwards.)
>
> --
> Ed Avis <eda at waniasset.com>
>

I have a Vista HCx, too. Bafflingly, Garmin does not store the DOP with the
tracklog and there is no way to extract it after the fact (or even realtime,
I don't think). If you can get an NMEA stream, you can get the HDOP (which
is what you care about for this application). The USB-only Garmin units
(like the Vista HCx) cannot emit a NMEA stream like the serial-enabled units
could. I believe the Garmin 60CSx has USB and serial and can emit NMEA over
the serial port, but you'd need a laptop to capture it. By the way, the
circle on your Vista HCx is a function of both your EPE (positional error as
estimated by the receiver) AND the resolution of the maps you're currently
displaying. The circle is more conservative (i.e., larger) than the
displayed EPE number, too.

Karl
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