[OSM-newbies] Trust GPS or Yahoo aerial imagery?

[Rick Collins]

At 03:57 PM 10/9/2007, you wrote:
>Rick Collins wrote:
> > I very much disagree with this idea.  Existing maps and images are
> > not likely to be consistently off by a significant amount.
>
>What makes you assert that? I think it's eminently possible that the
>orthorectification or alignment of the Yahoo images is out by 10m. 10m
>is not very far, when you have to figure out exactly where a speeding
>plane or satellite which took the photos is.

So how does this mean that GPS signals are more accurate?  The last 
time I checked, GPS calculations were done by explicitly calculating 
both the position and the speed of multiple satellites in a very high 
orbit, which means a very, very high speed compared to an aircraft or 
low earth orbit.


>They are also much more likely to be *consistently* off within a certain
>area than they are to be *inconsistently* off.

I won't dispute that.  My point is that I have observed GPS receivers 
to be inaccurate on numerous occasions.  I have even seen GPS 
receivers to read differently by 10 meters just by the simple act of 
turning them off and back on.  Clearly there is much room for error 
in a GPS reading or set of readings.  I also see significant 
variation between successive trips along the same route.  The 
distortions caused by variable ionization in the upper atmosphere is 
a significant source of error and often introduces a bias which 
varies slowly with time and little over a short path.


>My GPS on the other hand, is an OpenStreetMap recommended model, and I
>was walking up and down the centre of the roads (not cycling or driving)
>with a clear view of the sky.

I have no idea what is meant by "OpenStreetMap recommended 
model".  Are you trying to say that your model of GPS receiver is 
more accurate and consistent than others?  How was that 
determined?  What model is it?  How about checking your route by 
comparing to some other data rather than just making an assumption 
about which is more accurate and which is less accurate?  If you 
don't want to use the USGS images, why not try measuring the same 
route on multiple days?  If your data overlays itself well, then I 
would agree that it is likely that the imagery you are using is less 
accurate.

There is a site somewhere that provides a "solar weather" report.  I 
can't find my link to it, but I have seen a very strong correlation 
to actual GPS accuracy and solar activity.