How to render from Mapnik at a specific scale?

Question

I'm trying to render a map to paper using Mapnik at a specific scale (such as 1:50k), but can't yet figure out how. So far I'm just using map.zoom_to_box:

map.zoom_to_box (mapnik.Envelope (101.5,3.2,102,3.3))

and play with MaxScaleDenominator and MinScaleDenominator in the styling rules..

But when done this way, I can only guess-work the map scale by adjusting the bounding box over and over, which is hard to obtain the exact 1:50k scale that I wanted.. I've also found this: map.scale_denominator() and map.scale(), but I think these are used only to display the map scale - not setting it..

Is there anyone here who can help me?

Answer 1

I think I've found a workaround to my problem, but it's not a neat one. Basically:

1. I used print map.scale_denominator() to figure out the scale of my current selection bounding box
2. map.pan_and_zoom(+x,+y,z) to pan and zoom to the area that I wanted
3. Used print map.envelope(). This will give out the current bounding box used after panning: Box2d(minlong,minlat,maxlong,maxlat)
4. Adjust map.pan_and_zoom(+x,+y,z) z-values to zoom until map.scale_denominator() printout the scale value I wanted (50k)
5. When the map is panned at the right location, I just read the current map.envelope() values and transfer them into the map.zoom_to_box as the new bounding box. I'll comment out map.pan_and_zoom(+x,+y,z) when I'm done.

The overall code will look something like this:

map = mapnik.Map (3000, 3000)
mapnik.load_map (map, "mapDefinition.xml")
map.zoom_to_box (mapnik.Envelope (101.789315885,3.1540902527,101.977962094,3.3427364622))
map.pan_and_zoom(+1000,-100,0.568)
mapnik.render_to_file (map,"map.png")
print map.scale_denominator(), map.scale()
print map.envelope()

Like I said - messy. But it gets the job done.

Answer 2

This post should answer your question. Basically you should do a bit of math based on the resolution at which you want to print.