Just how accurate is TomTom satnav GPS positioning
TJ
Chingford, Greater London UK
- which is supposed to be 'Zero' degrees on Greenwich Meridian, at
1725 GMT, 28 Sept 08
It read N 51.63632 W0.00145 - not exactly ZERO degrees.
How innacurate is the reading?
Even Google Earth doesn't show total zero on the bench mark point.
The URL is for the map of Pole Hill ( which has line-of-sight to the
Royal Greenwich Observatory)
http://www.hill-bagging.co.uk/mountaindetails.php?qu=L&rf=903
Any ideas
TJ
Roger Mills
Anthony R. Gold <not-fo...@ahjg.co.uk> wrote:
> GPS uses the WGS-84 datum. I imagine that the Greenwich mark is on
> exactly 0W on the OSGB 36 datum (anyone know?). Some general purpose
> GPS receivers allow you to set different datums (actually they just
> translate from WGS-84 to a different display datum) but TomTom does
> not have that feature.
>
> http://en.wikipedia.org/wiki/British_national_grid_reference_system#Datum_shift_between_OSGB_36_and_WGS_84
> claims that places in England vary from 70m difference (WGS-84 being
> East of OSGB 36) in longitude in Cornwall to 120m in Eastern East
> Anglia.
>
> Tony
Sounds about right. 0.00145 degrees east/west at a latitude of 51 degrees is
just about 100 metres - which is an order of magnitude too high for a
positional error when the right datum is used, but in the right range for a
datum shift.
--
Cheers,
Roger
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Happy Trails
wrote:
>I put a Tomtom One UK on top of the BS benchmark 3276 on Pole Hill,
>Chingford, Greater London UK
>
> - which is supposed to be 'Zero' degrees on Greenwich Meridian, at
>1725 GMT, 28 Sept 08
>
>It read N 51.63632 W0.00145 - not exactly ZERO degrees.
>Any ideas
You have to check it at exactly noon or midnight if you want it to
work.
Mike Russell
> I put a Tomtom One UK on top of the BS benchmark 3276 on Pole Hill,
> Chingford, Greater London UK
>
> - which is supposed to be 'Zero' degrees on Greenwich Meridian, at
> 1725 GMT, 28 Sept 08
>
> It read N 51.63632 W0.00145 - not exactly ZERO degrees.
I did a similar check at Greenwich itself using an eTrex Legend. The prime
meridian was off by about 300 feet. The nice man at the entrance said they
would look into it.
--
Mike Russell - http://www.curvemeister.com
Uwe Hercksen
Mike Russell schrieb:
> I did a similar check at Greenwich itself using an eTrex Legend. The prime
> meridian was off by about 300 feet. The nice man at the entrance said they
> would look into it.
Hello,
something you should read:
http://en.wikipedia.org/wiki/Prime_Meridian
http://gpsinformation.net/main/greenwich.htm
http://www.nationmaster.com/encyclopedia/Prime-Meridian
http://www.gearthblog.com/blog/archives/2006/02/why_google_eart.html
http://www.flamsteed.info/faswgs84.htm
Bye
null
"Mike Russell" <grou...@MOVEcurvemeister.com> wrote in message
news:19mr9hzan2apt$.dlg@mike.curvemeister.com...
LOL!
David Mauro
Jack Yeazel
Uwe Hercksen wrote:
>
> Mike Russell schrieb:
>
> > I did a similar check at Greenwich itself using an eTrex Legend. The prime
> > meridian was off by about 300 feet. The nice man at the entrance said they
> > would look into it.
Since the second referenced site (below) comes from our website, I might
add that the reason WGS-84 (and GPS) 0� Long. are about 100m off the
Airy line in Greenwich is that WGS-84 selected where the Airy line
crossed the EQUATOR as being 0� Longitude...
Due to the difference in the British GB36 and WGS-84 datums, the two
lines diverge as one travels north from the equator...
> Hello,
>
> something you should read:
>
> http://en.wikipedia.org/wiki/Prime_Meridian
> http://gpsinformation.net/main/greenwich.htm
> http://www.nationmaster.com/encyclopedia/Prime-Meridian
> http://www.gearthblog.com/blog/archives/2006/02/why_google_eart.html
> http://www.flamsteed.info/faswgs84.htm
>
> Bye
--
Jack
Get general GPS information at: http://www.gpsinformation.net/
Uwe Hercksen
Jack Yeazel schrieb:
>
> Due to the difference in the British GB36 and WGS-84 datums, the two
> lines diverge as one travels north from the equator...
Hello,
marking the intersection of the equator and the prime meridian may
require a research submarine:
http://confluence.org/confluence.php?lat=0&lon=0
dropping a steel ball from a boat is not precise enough...
Bye
Jack Yeazel
Yeah, that's the standard 'gag'... I suppose it was all done with
mathematics...
However, the actual basics of WGS-84 zero Long. has to do with many
survey pins on all the continents... It is based on the AVERAGE
location of the land masses as they 'float' around the world... That's
why the call it "World"...
Surveying in the US is based on NAD-83 which is a datum that 'falsely'
assumes the US isn't drifting with respect to WGS-84... That way, Lat.
Long. of a particular point is more 'stable' than if based on WGS-84...
Here in Atlanta the difference is about 2m -within the range of
detraction with a quality hand-held unit and waypoint averaging...