rotate mapnik output map

mapnik-rotate

Mapnik supports the PROJ.4 WKT for projections and buried deep in the doc's is one called 
'Two Point Equidistant'. You provide the left and right points and the map is rotated 
such that these two points are horizontal in the output.
 
This allows you to rotate the map to any angle (including south up!!) and the text is 
rendered correctly! You don't have to change the projections of your input vectors. 
Simply tweak the mapnik generate_image.py script like this:

Change:

merc = mapnik.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 +nadgrids=@null +no_defs +over')

To:

merc = mapnik.Projection('+proj=tpeqd +lat_1=35 +lat_2=35 +lon_1=-80 +lon_2=-122  +x_0=0 +y_0=0 +ellps=WGS84 +units=m +no_defs')

And the map will render south up.

For an overlay raster I had transparent grid artifacts until I reprojected the image
to the target SRS. Like this:

gdalwarp -t_srs '+proj=tpeqd +lat_1=35 +lat_2=35 +lon_1=-80 +lon_2=-122  +x_0=0 +y_0=0 +ellps=WGS84 +units=m +no_defs' -co TILED=YES src.tif dst.tif

Then change the srs tag in the mapnik project xml for the raster (originally 
styled in TileMill):

<Layer name="raster"
  srs="+proj=tpeqd +lat_1=35 +lat_2=35 +lon_1=-80 +lon_2=-122  +x_0=0 +y_0=0 +ellps=WGS84 +units=m +no_defs">
    <StyleName>raster</StyleName>
    <Datasource>
       <Parameter name="file"><![CDATA[dst.tif]]></Parameter>
       <Parameter name="type"><![CDATA[gdal]]></Parameter>
    </Datasource>
</Layer>

The map then overlays the raster correctly, labeled, and with south up.

Very useful, thanks! The two points ("left" and "right") should be chosen well to avoid crazy distortion.