[Inkscape-user] How to set mm to px ratio

Hi,

Is there a way to specify mm to px conversion when exporting svg using inkscape --export-png? The --export-dpi doesn't help because it's overwritten by --export-width.

What I want is have one svg - map. The image will be exported for different zoom levels, so the pixel size of exported image will vary. But pixel width of roads should remain the same. In batik, I can specify road width in mm and then set different different mm to px ratio for each zoom level to get the same road width at all zoom levels. But I can't find such option in inkscape.

Is there some way or workaround how to do this in inkscape?

-- Jiri