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

[Lars Aronsson]

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.


-- 
  Lars Aronsson (lars at aronsson.se)
  Aronsson Datateknik - http://aronsson.se