[Talk-GB] Ordnance Survey 7th series available

David Earl david at frankieandshadow.com
Wed Oct 7 11:23:02 BST 2009

NPE maps have always had major alignment problems which have seemed to 
me to be worse in the eastern side of the country. There's also a new 
problem, but I don't know whether it is in the JOSM WMS plugin, the tile 
server or what.

Consider three JOSM screenshots:

The first is Landsat, obviously. It defaults with an offset of about 
100m vertically and horizontally in my part of the world (52N 0E) as can 
be seen from the obvious course of the road on the satellite images at 
the orange arrows. But once the layer is aligned on known points, it 
tends to be aligned across the whole working area.

NPE maps in contrast, in the second one, firstly has a relative 
displacement of tiles (about 150m) in JOSM which is obvious in this 
unadjusted download. (We also have to add to that that the map sheet 
boundaries don't line up anywhere near accurately)

The third one though shows that even within a single tile, the alignment 
drifts extremely rapidly. I aligned the third one with the junction at 
point 1 near the top left of a tile, and you can see that at the bottom 
right of the tile, the bend I've marked is off by about 150m 
horizontally and 115m vertically over a distance of about 1500m - 10%. 
All the roads in this area are GPS surveyed (I did them).

I mentioned this when NPE maps first arrived, but it's never been 
addressed. Is it a rectification problem, or a problem of how JOSM or 
the WMS plugin scales or displays the tiles?

The tile displacement problem is recent and I'm sure that will easily be 
fixed. The variation across a tile is more awkward I think.


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