[OSM-newbies] Consistently off tracks on second or third tracking

[Andy Robinson (blackadder)]

Lars Aronsson wrote:
>Sent: 07 February 2008 12:56 AM
>To: newbies at openstreetmap.org
>Subject: Re: [OSM-newbies] Consistently off tracks on second or third
>tracking
>
>Sorry for replying to an old message.  On January 21, Franc Carter
>wrote:
>
>> Out of curiosity, how do you know it's off, whenever I think
>> about this sort of thing I have a really hard time working out
>> what might be a good 'source of truth'.
>
>You build a very stable floor from concrete and mount a GPS
>receiver there.  Receive signals for a month and compute the
>average of the reported location.  Now you know the exact position
>of that concrete floor, with much higher accuracy than any single
>sample.  Next day, when satellites report "you are at position X,
>Y", you know how much off they are, because you know your own
>position much better than the satellite.
>
>Actually, what you do know is how much delayed the time signal
>from each satellite is.  If you can somehow (by GSM, WLAN, wire,
>27 MHz CB radio, or carrier pigeons) get this delay correction
>information out to your mobile receiver, it can compute it's
>correct latitude and longitude.
>
>The WAAS/EGNOS systems do exactly this and the correction
>information is sent out via geo-stationary satellites.  Since
>geo-stationary satellites orbit above the equator, if you are far
>north (in Scandinavia) you need a clear view of the southern
>horizon to receive this signal.  And your GPS device needs to be
>prepared/enabled for WAAS/EGNOS.
>
>But if you are a commercial, public or military surveyor, you
>might have access to other systems than WAAS/EGNOS.
>

Indeed, if you are a land surveyor you are probably using a GPS receivier in
some sort of DGPS mode, ie connected via phone or net to the correction data
pushed out by fixed base stations. UK Ordnance survey has an array of fixed
stations around the country so that you get the correction data correct for
your location. Its all done in real time and makes the difference between
our usual +/-10m with a handheld without WASS/EGNOS or +/-5m with them to
the +/- 3cm that the OS work to.

It is possible to use the same correction data (downloaded as historical
information after the event rather than in real time) in a post processed
way and improve our handheld GPS results, perhaps to provide accuracies in
the sub 5m range. However it's a lot of work for relatively little gain.

Check out http://www.ordnancesurvey.co.uk/oswebsite/gps/ for instance for
more on these things.

Cheers

Andy