On 2020-08-19 17:21, Russ Garrett wrote:
> On Wed, 19 Aug 2020 at 16:00, Colin Smale <colin.sm...@xs4all.nl> wrote:
>
>> At least it sounds soluble. Given the right transform and corrections a
>> "definitive" OS point in Easting/Northing format can be translated
>> accurately to WGS84 lat/long. However you look at it, I would expect a
>> purely mathematical transformation should have less error than a
>> transformation involving "tracing" from imagery whose rectification has
>> probably also involved some of these transformations each with their own
>> error terms. But I suppose that it at least partly depends on your
>> definition of "perfection."
>
> Well, that assumes that OS's locations are perfect, and that their
> data isn't subject to orthorectification errors and the like. It's
> still likely to be better than any other source, but I'd be surprised
> if there weren't similar errors in some of the OS data, especially in
> more rural & hilly areas.
Agree with this.
Here's a thought: I doubt the OS use equipment that reads out directly
in OSGB36. I would think it it more likely that their equipment provides
(e.g.) WGS84 data which is then converted to OSGB36 before storing it in
the MasterMap database? I wonder what transformation errors are
introduced at that stage. If we reverse that transformation exactly, we
should arrive at the data as originally captured.
> In my experience, once you start trying to go below 5m accuracy you
> swiftly learn not to trust anyone.
I can understand that, but it is not helping us forward... We need to
trust someone/something unless we think we can do better ourselves. I
tend to regard the OS as "sufficiently trustworthy" from my perspective
as a reasonably technically, mathematically competent person without any
specific inside knowledge of the OS. So if the OS say it's at {x,y} and
some other source says {x',y'} then I would presume that the OS is more
likely to be correct, and the other source would have to show me a damn
good provenance if it wants me to consider it better than the OS (and
thereby prompt me to move some feature in OSM)._______________________________________________
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