I've seen this particular problem with this projection and shapefile
numerous times.
The reliable way I've found to deal with it has been to remove
Antarctica. =\
-mike.
On Oct 13, 2008, at 3:51 PM, Jon Burgess wrote:
> On Mon, 2008-10-13 at 18:24 -0400, Filipe Fortes wrote:
>> Hello All,
>>
>> I'm just starting out with Mapnik and having an issue trying to
>> duplicate the mercator projection used in Google Maps.
>>
>> I've taken the Hello World example from the map and modified it to
>> use
>> a different projection:
>>
>> #!/usr/bin/env python
>> from mapnik import *
>> projection = '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0
>> +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m [EMAIL PROTECTED]
>> +no_defs +over'
>> m = Map(600,300,projection)
>> m.background = Color('steelblue')
>> m.srs = projection
>> s = Style()
>> r = Rule()
>> r.symbols.append(PolygonSymbolizer(Color('#f2eff9')))
>> r.symbols.append(LineSymbolizer(Color('rgb(50%,50%,50%)'),
>> 0.1))
>> s.rules.append(r)
>> m.append_style('My Style',s)
>> lyr = Layer('world')
>> lyr.datasource = Shapefile(file='shapefiles/world_borders')
>> lyr.styles.append('My Style')
>> lyr.srs = '+proj=latlong +datum=WGS84'
>> m.layers.append(lyr)
>> p = Projection(projection)
>> lower_left_xy = p.forward(Coord(-180.0, -45.0))
>> upper_right_xy = p.forward(Coord(180.0, 80.0))
>> bbox = Envelope(lower_left_xy,upper_right_xy)
>> m.zoom_to_box(bbox)
>> render_to_file(m,'images/world.png', 'png')
>>
>> I've attached the output -- as you can see, it's only rendering a
>> subset of the countries, and I can't find any rhyme or reason why
>> only
>> those countries are showing up.
>>
>> I've also switched and used the TM world borders shapefile [1] --
>> which renders exactly the same with the exception of also rendering
>> the US. If I go back to using the latlong projection, this problem
>> disappears.
>>
>> I'm guessing there's something basic I'm missing here, so I
>> appreciate
>> any one taking the time out to help a newbie :)
>
> That looks like the sort of issue that can happen when you deal with
> projecting data at or very close to the 'edge' of the world. Problems
> happen when the extremes of the bounding box in one projection wrap
> when
> projected to another. I had this with OSM shapefiles:
> https://lists.berlios.de/pipermail/mapnik-devel/2007-September/000236.html
>
> Try:
> - reducing the +-180 in the Coord() to say +-179
> - check the extent on the shapefile (shpinfo will give you this)
>
> You may need to clip the edges of your shapefile very slightly (again
> try +-179 to see if that helps). 1 degree is overkill but should
> demonstrate if this helps.
>
> The more reliable solution is to convert your shapefiles to match the
> map projection and then use these in Mapnik. Avoiding the reprojection
> during the rendering is key.
>
> Jon
>
>
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michal migurski- [EMAIL PROTECTED]
415.558.1610
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