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I would like to create a world map "upside down", with the South on top (in Mapnik).

How can I achieve this?

Can I take an existing projection string, e.g. Robinson projection, ESRI:54030.

+proj=robin +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs

and flip/mirror it? So the North will point down.

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In PROJ.4 trunk, there is upcoming support for 'upside down' Mercator projections in Proj.4, which Frank explains how to use as such:

"proj trunk" does add new commandline arguments for switching around the axes that might be sufficient for your mirroring needs. If you can build with trunk, you could try adding something like:

+axis=esu

which means that the X axis is "easting" the Y axis is "southing" and the Z axis is "up".

Alternatively, you could do the rotation after the projection step, which seems to be what Mapserver has implemented.

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