Mapnik UTFGrid Provider.
Takes the first layer from the given mapnik xml file and renders it as UTFGrid
It can then be used for this:
Only works with mapnik>=2.0 (Where the Grid functionality was introduced)
Use Sperical Mercator projection and the extension "json"
mapfile: the mapnik xml file to load the map from
fields: The fields that should be added to the resulting grid json.
layer_index: The index of the layer you want from your map xml to be rendered
wrapper: If not included the json will be output raw, if included the json will be wrapped in "wrapper(JSON)" (for use with wax)
scale: What to divide the tile pixel size by to get the resulting grid size. Usually this is 4.
buffer: buffer around the queried features, in px, default 0. Use this to prevent problems on tile boundaries.