[Talk-GB] bing image alignment

[SK53]

AFAIK all OS StreetView tiles on OSM are projected in Spherical Mercator
and this reprojection will certainly have been done using proj4, i.e, it's
algorithmic not table-driven.

OSSV scale is 1 pixel / metre, so accuracy is less than that.

So sources of error are:

   - Feature generalisation in OSSV, noticeable on buildings & roads
   - Re-projection of OSSV tiles using proj4 using OSGB36, errors of +/- 5m
   - Known GPS signal errors (LDOP)
   - Unknown & systematic GPS errors
   - Fudge factor in aligning these two sources
   - Human factors in interpreting alignments

Taken together this suggests 6-10 metres is entirely in the range that we
might expect & that keeping topological relationships is more important.
Given an average road is about 7-8 metres across we are less likely to
notice this with roads anyway.

Obviously an aerial imagery source introduces a number of other error
sources.

If one looks at GPS traces for the same footpath walked again and again,
their spread is quite considerable, perhaps as much as 20 m, although an
average would probably be close to the actual 2 m path.

So just like any other survey organisation, ultimately we will need our own
set of convenient reference points.

Jerry


On 13 September 2013 14:37, OpenStreetmap HADW <osmhadw at gmail.com> wrote:

> On 13 September 2013 14:22, Colin Smale <colin.smale at xs4all.nl> wrote:
> > Cm-level GPS accuracy is coming within our grasp... My attention was
> > recently drawn to this:
> >
> > http://www.kickstarter.com/projects/swiftnav/piksi-the-rtk-gps-receiver
> >
>
> You need to operate in differential mode to get the full accuracy,
> which means you still need an accurately surveyed datum.
>
> I don't see that it will help with reflections.
>
> I also wonder if it will prompt the re-introduction of selective
> availability, as it gives some of the advantages of the higher
> chipping rates on the, encrypted, find mode GPS signal, and probably
> doesn't have the velocity and altitude caps normally applied to
> commercial receivers.  I believe the military have the advantage that
> they can use the second frequency to help do large area ionosphere
> corrections.
>
> _______________________________________________
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>
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