[OSM-dev] Specifying area and zoom in Mapnik

[Artem Pavlenko]

On 23 Mar 2007, at 15:52, Keith Sharp wrote:

> Hello,
>
> I've got Mapnik installed and rendering data from my PostGIS  
> install of
> the weekly planet.osm if I use the generate_tiles.py script from  
> svn.  I
> can modify the area and zoom level I want and it all works.
>

> What I would like to do now is create a script where I can specify an
> image size, and the area I want to render.  I am having difficulty
> working out how to do this.  I got as far as:
>
>
> #!/usr/bin/python
>
> from mapnik import *
>
> mapfile = "/home/kms/src/osm/mapnik/osm/osm.xml"
> m = Map(800, 800, "+proj=latlong +ellps=WGS84")
> load_map(m, mapfile)
Here srs gets  overwritten with one form mapfile. I suspect you have  
mercator in osm.xml

> bbox = Envelope(Coord(-50.0, -50.0), Coord(50.0, 50.0))

Given the above is true you need to specify coordinates in Mercator  
projection

p = Projection("+proj=merc +datum=WGS84")
c0 = p.forward(Coord(-50,-50))
c1 = p.forward(Coord(50,50))
bbox = Envelope(c0,c1)

HTH

Cheers,
Artem



> m.zoom_to_box(bbox)
> render_to_file(m, 'map.png', 'png')
>


>
> But this doesn't do what I expect.  My main question is how do I  
> specify
> the bbox as a pair of WGS84 kat/lon coordinates?
>
> For example I'd like to render the entire UK as:
>
> 	-8.920, 48.900 to 2.250, 61.300
>
> or the area around Glasgow West:
>
> 	-4.350, 55.860 to -4.290, 55.900
>
> Any help would be appreciated!
>
> Keith.
>
> PS. Regarding getting the tile rendering stuff up and running - I plan
> on tidying my notes up and putting them on the Wiki.
>
>
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>

Artem Pavlenko
http://mapnik.org



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