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Let's say we have mercator map with given resolution (for simplicity in centimeters).
Now let's get coordinates of (lat,lon) = (50, 15):
$ proj +proj=merc +ellps=WGS84 50 15 5565974.54 1678147.52
Since we don't know the boundaries, we can't scale it to desired resolution.
The only way I've found to get boundaries is compute the coordinates for (lat, lon) = (180, 0) and (0, 90), but that's more like a hack and it's not universal (other projections might map only from (±90, ±90)).
Does proj4 deal with scaling differently?