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

[Andy Allan]

On 10/9/07, Rick Collins <gnuarm.2006 at arius.com> wrote:
> At 09:01 AM 10/9/2007, you wrote:
> >Richard Fairhurst wrote:
> > > To answer the original question, though, trust your GPS over the
> > > imagery. The imagery may not always be rectified, whereas as long as
> > > your GPS has produced a good-quality track - i.e. no "concrete
> > > canyon" distortions - it should always be accurate.
> >
> >OK. So if that's the answer (and it's sort of what I expected) then the
> >follow-up question is: do we have any procedure for fixing the
> >rectification, or letting Yahoo know their images are out?
>
> Maybe you guys should take a step back and look at what you are
> saying.  You have one person with a $200-$400 GPS receiver recording
> a track which then is hand edited to add to an open source map.  This
> track disagrees with aerial imagery that companies have spent
> millions or even billions of $$$ to collect, process and
> provide.  Now you are thinking of telling a distributor of that data,
> based on your single track of collected data, that they need to
> "correct" their data.

Yes, actually. It's well known that the imagery is not precisely
orthorectified, but it's usually acceptably close. We've seen this
happen in many areas, backed up by statistically significant numbers
of GPS tracks from different receivers over huge amounts of time. The
most common thing is to be a few tens of meters off in a particular
direction e.g. the images have the junctions 15m NE of their true
location.

Remember they haven't spent "billions" of dollars on each image tile,
so it's expected some of them will be a bit squiffy.

> Do you really think they are going to take you seriously?  Perhaps
> you are thinking of the Yahoo imagery as if it were a collaborative
> effort like OSM?

We've got a lot of experience, collectively, at this game nowadays,
and we know what we're talking about.

Cheers,
Andy