What is the correct projection I should use with Mapnik?

Question

Hello,

I'm trying to render tiles with a very simple stylesheet. For data feed I'm using OSM file extract from the planet dump. I've read Mapnik can handle OSM XML files with some limitations.

There are two problems:

Long ways are not rendered.
All ways are distorted or displaced.

You can see the result here: http://dev.ivanatora.info/roads/ At least the big roads in these two layers "Roads" and "Roads tiles" should match.

The second one seems like a projection problem and I tried tons of different variants but in vain. Here is my latest stylesheet: http://dev.ivanatora.info/tests/mapnik_style.xml I've tried also simplier things for srs - "+proj=merc +datum=WGS84" and "+proj=latlon +datum=WGS84". I think the data from the planet dump extract is in latlon projection, but something is not right...

I would really appreciate some insight.

Accepted Answer

When rendering with Mapnik, you have an input projection and an output projection. The input projection is what your data is in, and has to be given in the <Layer> element. The output projection is what you want your map to be in, and is usually given in the <Map> element (but some client programs e.g. nik2img.py allow you to override that).

You likely want your map in Google Mercator projection so your <Map> element

<Map bgcolor="transparent" srs="+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 +nadgrids=@null +no_defs +over">

looks ok (but try writing srs="+init=epsg:3857" which should also work and is clearer).

Your <Layer> elements claims that the source data is in Mercator as well, but it isn't; this will lead to distorted rendering. Your source data, raw latitudes and longitudes from OSM, is in EPSG:4326, so you'll need something like this:

<Layer name="highways" status="on" srs="+init=epsg:4326">

What is the correct projection I should use with Mapnik?

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