[OSM-newbies] difference between yahoo WMS and gps track

[john whelan]

There is a technique called averaging which basically says that if you
take the average of a number of inaccurate readings you answer will
probably be more accurate.  I say probably because its one of the math
things.

Each reading is probably within 4 meters however if you take 100
readings then the majority of those readings will actually be within 2
meters accuracy take the average and its just about spot on.

GPS tends to vary a bit according to the weather so recording at
different times on different days helps.  Reflections from buildings
and lack of line of site mean GPS devices are not too accurate in city
centers or near tall buildings.  Most city roads are fairly straight
so take readings at the ends of roads in cities away from buildings
and draw in straight lines after wards.

GPS is usually more up to date.  New features can be added in
directly, OSM is not so much about physical accuracy but about
relative accuracy.  If everything drawn on the map is 5 meters out it
doesn't matter that much.  Satellite data can be slightly out, tracing
can introduce more errors.

Cheerio John



On 5 February 2010 09:16, Jonas Stein <news at jonasstein.de> wrote:
> Both datasources have their weaknesses. I want to understand the
> errorfunction of both better to get most out of the data.
>
> Lets assume
> The Garmin 76SCx tracks with 1 Point/s it shows +/-4 m in Display (1)
> WAASG and EGNOS is enabled. Compass = ON
>
> The track shows in JOSM as a very smooth line. Walking on one other side
> of the street back shows a parallel dotted line like . and '
> Then i draw a way in the middle the WMS pendant is shifted
> 10 to 30 metres but has same shape.
>
>
> ======================
>
>
> ......................->
> ------------------------
> <-''''''''''''''''''''''
>
>
> =  wms
> .' GPX Track
> -  mean line
>
>
> 1) where does this information come from?
> 2) has the gpx track an offset?
> 3) are there any tricks to get rid of that difference locally?
>
>
> When i walk a circle with the gps the data forms a very precise circle.
>
> Kind regards,
>
> --
> Jonas Stein <news at jonasstein.de>
>
>
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