[OSM-talk] NPE map WMS server projections

[Robert (Jamie) Munro]

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Nick Whitelegg wrote:
>> This is probably one for Nick.
> 
>> The WMS server that runs on nick.dev.openstreetmap.org that serves up
>> NPE maps needs adjusting to as to reproject each individual tiles to
>> match the grid squares, and reproject/reorient the grid square corners
>> to suit the WMS.
> 
>> This is obvious when you start looking at Cornwall or West Wales, where 
>> the OS grid north is nowhere near true north, and the corners of the grid 
>> squares are more than 50 metres out.
> 
> Can anyone suggest any guidelines as to how I would go about correcting 
> this (as, I have to admit, I'm a bit unsure?) At the moment the WMS server 
> just "glues together" the appropriate NPE tile images based on the 
> bounding box, width and height. I guess, therefore, that this is an 
> inherent problem in the accuracy of the NPE tiles.
> 
> Would it involve something like "twisting" the tile so the longitude of 
> the top and bottom are equal?

It would. You can see the problem illustrated if you have Google Earth,
turn on Lat/Lon lines in the menu, and load the grid.kml file here:
http://www.arjam.net/google.html

I found it a useful way to understand why the OS grid is different to
the lat/lon grid - why grid north isn't the same as true north. It's not
because they made a mistake measuring it years ago and we are stuck with
it. :-)

Robert (Jamie) Munro
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