[OSM-talk] quality of drawn way

[Andrew Rowbottom]

Ho hum, just some rambling thoughts.....


I've had a bit of fun lately playing with mapserver (a WMS server)
reprojecting maps. It really shouldn't be too hard to get it serving
NPE maps up in mercator or WGS84.

I seem to remember once upon a time the code in JOSM attempts to
re-project from OSGB to WGS84/Mercator it does a fair job, but doesn't
do rotations which can be noticeable in large parts of the country
e.g. Cornwall. Tis been mentioned previously on this list. The actual
data to JOSM appears to come down in OSGB projection which is why JOSM
has to do the re-projection.

I do have a php script somewhere that re-projects from the NPE tiles
to WGS84, and another one that can serve a WMS like request from a set
of tiles in google maps format, both of these scripts are messy, slow
and inefficient. Mapserv appears to be more efficient at the job.


1. Not all the tiles are actually cut on the grid lines.
2. The OS didn't make all grid squares equal, the ones at the top and
bottom typically seem to be taller which has to be adding systematic
errors in somewhere.
3. I'm not convinced that the OS just post war was as accurate as we
can be with GPS's nowadays. As Mike said Locally accurate, inaccurate
over a few miles, accurate over 10s of miles. I'm glad Mike has seen
this because I've had issues with accuracy on another old maps site,
and it's nice to have it confirmed.

Anyway IMHO Point 1 isn't that big a deal because of points 2 and 3.
Because of this I never use the NPE maps blindly, I always make sure
there's a reasonable GPS trace in the area to line things up properly.
I personally would advise others to do likewise.



Andrew Rowbottom