Torsten Mohr wrote:
> I understand that the 'Datasource' (e.g. PostGIS) contains the logical
> information that make up a map, so to say where is a street, where is
> a river, right?
Yes, the PostGIS db contains geometry data, and other columns with data
that's used in rules to determine what data to render.
> A 'Style' tells how to graphically visualise a certain category of data, e.g.
> to draw a river in blue, a forest in green and, is that right?
Where you define a layer to tell mapnik *which* data to fetch from the
database, a style is used to tell mapnik *how* and *when* to render that
data.
> I'm unsure about 'Rule', does it describe when to render a certain category?
> Something like "don't render streets if we look at the whole world"?
Among others. The example you gave is decided by the
Min/MaxScaleDenominators. Other things that are normally in a rule are a
filter ("render the data if it passes this filter") and the actual
drawing rules ("draw a dotted line this thick and in this colour").
A style can and usually does contain more than 1 rule, for various zoom
levels.
> 'Layer's are used to distinguish certain categories, it is more for the
> application to order categories, right?
The layer order decides in what order the elements are drawn, from the
first defined layer in the stylesheet right up to the last, each
successive layer overwriting elements in previous layers. Search for
Painter's Algorithm on wikipedia.
> Is my understanding right or is there something missing?
No, you're on the right track.
> As i needed to download world_borders and also coast_line i guess
> these data sources are not contained in the huge "planet.osm", right?
Correct.
See also: http://trac.mapnik.org/wiki/XMLConfigReference
--
Lennard
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