[OSM-talk] New Popular Edition in JOSM

[Nick Burch]

On Sun, 12 Nov 2006, David Earl wrote:
> If I move the NPE maps to align on a key point, then they quickly 
> diverge in all directions from the point.

As a general rule, a given NPE calibration is only valid for about 2-4km 
from that point. It is also only valid on a given sheet - if you cross 
sheets, it'll probably be wrong

When tracing to a gpx (http://gpx.npemap.org.uk/), calibrating on grid 
square joins, I do find that the error changes as I move a few squares.

> Should I be worried about all the data I've entered? Can I use the NPE 
> layer for anything useful with any confidence?

I'd try tracing it to a GPX file (having calibrated!), and comparing that. 
You might just be unlucky, and be using a map that was distinctly off-flat

Nick