[Talk-GB] OS 7th Series offset

Andy Robinson (blackadder-lists) ajrlists at googlemail.com
Thu Oct 8 09:06:58 BST 2009

Richard Fairhurst wrote:
>Sent: 07 October 2009 5:34 PM
>To: talk-gb at openstreetmap.org
>Subject: Re: [Talk-GB] OS 7th Series offset
>> > 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.

No how about I ask the OS under a FOIA request to provide the error
correction values (rotation and position) for each of my 1:25k sheets. I
just wonder if they would play ball. I'll send one in once the current one
is back. Don't wish to taunt them too much :-)



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