[Talk-GB] OS 7th Series offset
richard at systemed.net
Wed Oct 7 17:34:07 BST 2009
> > I get a much better alignment for that area with the re-rectified
> NPE tiles
> > from TimSC's space at http://dev.openstreetmap.org/~timsc/wms2/map.php?
> Yes, indeed, vastly better (and the images are clearer too, and the
> tiles braking up problem isn't there). Even if I pan several tens of
> km it is still in sync, and it seems to be in sync from the download
> - no initial adjustment needed.
I rectified the NPE images available in Potlatch (and later
republished via Tim's WMS for those of you using inferior editors ;) )
with 90 control points per sheet. It took bloody ages! But it does
mean they're very, very close to the originals.
The original NPE images found at www.npemap.org.uk, and on the older
WMS, had only been rectified with some rough whole-sheet rotation and
shearing operations in Photoshop or similar. These were in fact
rectified for postcode input at npemap.org.uk, where the accuracy was
Personally I'm a bit tired of fixing JOSM users' crappy tracings and
would like the old WMS to be removed!
But the fact that even the Potlatch 90-point NPE tiles need a little
manual correction to GPS (space-drag in Potlatch) tells you something
about the OS's surveying accuracy back then. As the 7th series
followed straight on from NPE with no major resurvey at first, you
should expect there still to be some displacement.
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