Comparison between zoom levels and the scale denominator numbers needed for
the Mapnik Map file.

zoom level=1 scale_denom=279541132.014
zoom level=2 scale_denom=139770566.007
zoom level=3 scale_denom=69885283.0036
zoom level=4 scale_denom=34942641.5018
zoom level=5 scale_denom=17471320.7509
zoom level=6 scale_denom=8735660.37545
zoom level=7 scale_denom=4367830.18772
zoom level=8 scale_denom=2183915.09386
zoom level=9 scale_denom=1091957.54693
zoom level=10 scale_denom=545978.773466
zoom level=11 scale_denom=272989.386733
zoom level=12 scale_denom=136494.693366
zoom level=13 scale_denom=68247.3466832
zoom level=14 scale_denom=34123.6733416
zoom level=15 scale_denom=17061.8366708
zoom level=16 scale_denom=8530.9183354
zoom level=17 scale_denom=4265.4591677
zoom level=18 scale_denom=2132.72958385

scale_denominator = map_width_in_metres/ (map_width_in_pixels * 
standardized_pixel_size/*0.28mm*/)