[Talk-GB] More walking maps.

[Derry Hamilton]

Hi All,

On of the things I'd been after was a usable replacement for the
Landranger maps, including contours and a compatible gridding system.
Now, getting a perfect replacement has long been considered to be a
lot of work, and after looking at it, and getting to grips with OSTN2,
I figured that perfect wasn't something I actually wanted to do.  So I
decided to see how much work Good Enough would be.

The first thing was to decide what constituted good enough.  Obviously
that's subjective, so if you want to cut to the chase, have a look at
http://osm.rasilon.net/?zoom=13&lat=7527193.6868&lon=-366400.89503&layers=BTT
Please bear in mind that that host doesn't have the horsepower or disk
space for production use.


The basic additions I wanted were contours that gave a decent enough
landform, and for that SRTM data seemed good enough.

The grid was the next thing.  I wanted a usable grid that allowed me
to use six figure grid references with no significant loss of
accuracy.  Some of the military maps I've used for bits of the world
outside the UK have just projected the
http://en.wikipedia.org/wiki/Military_grid_reference_system grid over
whatever map they already had, and I found that there wasn't a whole
lot of usability lost.  So I went for that approach, creating a layer
that just projected an out-of-copyright OSGB36 grid over an OSM base.
A look at the differences between the original and OSTN2 corrected
suggests that the maximum offset is ~120m, so a six-figure grid
reference has an additional error of +-1 in the least significant
figure, which is close enough, I think, given that most of the ones
I've used were only approximately that good anyway.  Comparing the
results to Landranger maps has convinced me that it's close enough for
my purposes anyway.

The current config is mapnik/renderd pointing at a Postgres/PostGIS
database and mod_tile doing the serving.  Things like the style XML,
grid SQL script etc. are available if anyone is interested.

I think the next thing up is to try to get some of the Landform data
working in place of SRTM since there are problems with voids in some
cases (see http://osm.rasilon.net/?zoom=14&lat=7802921.15703&lon=-692519.47615&layers=BTT
) and this post was inspired by talk of other people getting it
working.

Have fun,
Derry