[OSM-talk] New Popular Edition in JOSM

[Andrew Rowbottom]

Ahoy,

On Sunday 12 November 2006 19:28, Nick Whitelegg wrote:
> > But now I'm a bit worried. The tracks in my area don't align. (And no,
> > it's not changes to the street pattern!) If I move the NPE maps to align
> > on a key point, then they quickly diverge in all directions from the
> > point.
>
> I'd be surprised if it's that bad - maybe it's an issue with that
> particular NPE map. Earlier today I got good alignment in an approximately
> 5km x 5km area on one NPE tile, and 2km x 2km on another.

There was a post on the ordnance maps list on yahoo which quoted a
reply from the OS, one part which caught my eye was :

> a programme of Positional Accuracy Improvement has been
> undertaken to address inconsistencies resulting from the recasting of
> the large scale Victorian County Series mapping onto the National
> Grid and the Overhaul of these maps during the post-war revision.

I might be wrong, but it looks like this could be interpreted as
meaning that, in some rural areas at least, the local scale positional
accuracy of current OS maps is, in effect, based on that achieved in
the last part of the 19th Century.

If this is true this would probably have been an even more common
occurrence in early NPE maps. Not being a map historian I can't be
sure of how big an effect this was, though I  can say with reasonable
certainty that in the previous series (Popular Edition 1920-30's) even
after ortho-rectifying on a 2mile grid and reprojecting some of the
mapped trig pillars can be adrift by up to 200m.

Andrew Rowbottom