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I've set up a tile rendering system based on Mapnik + PostGIS. When I open the data with QGiIS directly from PostGIS it looks fine; but when I attempt to generate tiles with Mapnik they look stretched (notice that this issue only appears when the rendered tiles are on another horizontal line).

Tiles viewed with OpenLayers: Tiles viewed with OpenLayers

Tiles viewed with QGISTiles viewed with QGIS:

Raw data from PostGIS viewed with QGIS: enter image description here

Can you please guide me trough what things should I check in order to fix this issue?

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I've found explanation here: https://help.openstreetmap.org/questions/13250/what-is-the-correct-projection-i-should-use-with-mapnik?

And solved the issue by setting "input" srs-attribute for layer:

<Layer srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs" name="land">

And "output" SRS for map:

<Map 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">

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