I have a local directory full of geotiff files which make up a map of the UK. 
I'm using mapnik to render different images at various locations in the UK. 
I'm wondering what is the best way to approach this?
I can create a single RasterSymbolizer then loop through the tiff directory and add each tiff as a seperate layer, then use mapniks zoom_to_box to render at the correct location.
But would this cause the rendering time to be unnecessarily slow? I have no information on how the tiles fit together (other than the data in each individual tiff of course).
I imagine there may be a way to setup some kind of vector file defining the tiff layout so I can quickly query that to find out which tile I need to render for a given bounding box?
You can either generate a big tiff file from the original tiffs with gdal_merge.py (you can find it in the python-gdal package on Debian or Ubuntu) or create a virtual file that mixes them all with gdal_merge-vrt. This second option saves space but probably is slower.
mapnik and local tiff tiles