[OSM-talk] simplifying mapnik layout definition

[Andreas Barth]

Hi,

currently the mapnik layout definition is a bit complex. Please consider
this example:
<Style name="tunnels-casing">
    <Rule>
      <Filter>([highway] = 'motorway' or [highway]='motorway_link') and ([tunnel] = 'yes' or [tunnel] ='true')</Filter>
      <MaxScaleDenominator>200000</MaxScaleDenominator>
      <MinScaleDenominator>100000</MinScaleDenominator>
      <LineSymbolizer>
        <CssParameter name="stroke">#506077</CssParameter>
        <CssParameter name="stroke-width">3</CssParameter>
        <CssParameter name="stroke-dasharray">4,2</CssParameter>
      </LineSymbolizer>
    </Rule>
    <Rule>
      <Filter>([highway] = 'motorway' or [highway]='motorway_link') and ([tunnel] = 'yes' or [tunnel] ='true')</Filter>
      <MaxScaleDenominator>100000</MaxScaleDenominator>
      <MinScaleDenominator>20000</MinScaleDenominator>
      <LineSymbolizer>
        <CssParameter name="stroke">#506077</CssParameter>
        <CssParameter name="stroke-width">5.5</CssParameter>
        <CssParameter name="stroke-dasharray">4,2</CssParameter>
      </LineSymbolizer>
    </Rule>
[ 2 more cases ]


The differences are only within the scales. So it would make sense to
split the scales off - and make them as small as possible. The smaller
it is, the easier one can make changes. So this could be written as:

<Style name="tunnels-casing">
      <Rules filter="([highway] = 'motorway' or [highway]='motorway_link') and ([tunnel] = 'yes' or [tunnel] ='true')"/>
      <CssParameter name="stroke">#506077</CssParameter>
      <CssParameter name="stroke-dasharray">4,2</CssParameter>
      <ScaleDenominator max="200000" min="100000">
        <CssParameter name="stroke-width">3</CssParameter>
      </ScaleDenominator>
      <ScaleDenominator max="100000" min="20000">
        <CssParameter name="stroke-width">5.5</CssParameter>
      </ScaleDenominator>
    </Rules>


This drops the (IMHO unnecessary) LineSymbolizer (one can make it up
from the stuff around it) and emphasizes the differences. If this looks
ok-ish, I could write an converter from the new to the old format as an
starter.


(Long-run should be to use parameters IMHO, so that we can just say
"stroke-width has that size in relation to the scale" - but that's for
another day)



Cheers,
Andi
-- 
  http://home.arcor.de/andreas-barth/