[Talk-GB] reprojected NPE

Richard Fairhurst richard at systemeD.net
Thu Apr 10 00:37:29 BST 2008

Tim Sheerman-Chase wrote:

> It may be worth reassembling the NPE map (from tiles or backup)
> and retiling, it but it would be a big job! (The potlatch blog  
> mentioned anchor points and improving alignments...)

I'm plotting anchor points on 5km x 5km squares which is working  
pretty well. You still need to do a little manual shifting for  
alignment, but this is mostly due to inaccuracy in NPE itself rather  
than in the reprojection.

> The whole thing is implemented in PHP and Imagemagick. It takes  
> about 60 seconds to render one 0.1 by 0.1 tile - pretty slow! The GD  
> library is much faster but it does not seem to have an affine  
> transform function.

Ah, ok. I'm using Perl and Imager, you're not even limited to affine  
transforms with that. :)

> I did see you are making NPE available on Potlatch. What projection  
> is this in? I am guessing spherical Mercator.

Yep, standard Google-like spherical Mercator tiles. They're at

where 'z' is 14 only at the moment, and x and y are Wales! (But growing.)

Is there some way in which JOSM can display these tiles?

> Is the 300dpi data available on the dev server or perhaps elsewhere?

The originals are way too big to put anywhere, I'm afraid. But the  
data at the address above is barely reduced in resolution and is a lot  
more manageable.


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