[Talk-GB] OS 7th Series offset

Richard Fairhurst 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  
less critical.

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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