[OSM-talk] New Popular Edition maps, and postcode collection

[Dominic Hargreaves]

On Tue, Oct 24, 2006 at 03:09:49AM +0200, lars wrote:

> Great work!  But the map doesn't come out quite as detailed as I 
> had hoped and it looks a little unsharp.  In particular the 
> sharpness seems to vary with folds and bends in the map, e.g.
> http://www.npemap.org.uk/tiles/map.html#323,206,1
> http://www.npemap.org.uk/tiles/map.html#495,123,1
> 
> What technology did you use to digitize the map?  Did you document 
> the process?

I expect that Richard can explain how the scans were produced. The tile
production roughly went as follows:

- Open scan of map sheet in the Gimp.
- Rotate the map so that the grid lines are horizontal/vertical
  (generally a 1-3 degree rotation)
- Apply a trapezoid transform if necessary to get the edges of the map
  (as close to as possible) parallel.
- Crop the image to just the map (removing the borders)
- Scale the image to 5000x5625 pixels (so each grid square becomes
  125x125 pixels). The original scans, including the borders, were
  around 5500x6500, +- a few hundred pixels.
- save as JPEG
- split into tiles with a bit of perl and Image::Magick
- rename the tiles into their final location with some more perl

The rotation, trapezoid and scaling will have all contributed to the
deterioration of the image, but was necessary to accurately geocode the
map (and indeed to display it on a tile-based display system).

I would estimate that this process took us around 15-20 man-hours for
the maps that we have.

Of course, as explained in the licence, the maps weren't pristine copies
to start with; some of them were quite worn, had markings on, or were
not very flat.

Cheers,

Dominic.

-- 
Dominic Hargreaves | http://www.larted.org.uk/~dom/
PGP key 5178E2A5 from the.earth.li (keyserver,web,email)