Landcover is not visible at z5 in finely-mapped areas

[jeisenbe]

Expected behavior

• The size of landcover areas should not affect the final rendering

Actual behavior

• Areas such as the Netherlands, where farmland and meadows are mapped as small parcels, appear blank at z5 and are lighter than they should be at z6 and z7
• This is because the pixel size limit is AND way_area > 0.01*!pixel_width!::real*!pixel_height!::real for low-zoom landcover, so any area smaller than 0.1 x 0.1 pixel height and width will not be considered. At the equator this is 61 x 61 meters at z7, and 246 x 246 meters minimum size at z5. At 60 degrees the limit is 30 x 30 meters at z7 and 123 x 123 meters at z5 - most farm and meadow parcels are smaller than this in the Netherlands.
• Also, Mapnik is not optimized for rendering many small polygons in one pixel, so even if this limit were changed it is likely that the rendering would still not be the same color as expected if a pixel consists of many small squares of meadow or farmland.
• The solution would require pre-rendering landcover in mapnik at a lower zoom level (as in the French fork) or pre-processing the polygons into larger areas.

Links and screenshots illustrating the problem

z5 Netherlands, Belgium, Norther France

• Netherlands appears unmapped, northern France looks better because landcover polygon size is much larger on average in France
  

z7 Northern France

• Farmland areas are fairly consistent at #F1F3E0
  

z7 Northern Netherlands

• Farmland areas are lighter - variable around #F5F5E4
  

But these images show that landcover is almost completely mapped (in meadow/grass/farmland) for these areas:
z9 Northern Netherlands

z13 Fryslan

z13 Groningen

z13 northern France

[jeisenbe]

Should we remove the low-zoom landcover from z5, z6 and z7 for now, pending a better rendering solution? I would of course like to rendering landcover even at lower zoom levels, provided it can be done a a sensible way.

[imagico]

I think with the landcover fading and the inconsistent low zoom use of patterns for landcovers (#3862) it is not really sensible to think about a real solution to this. Consistently starting the landcover rendering at a certain zoom level (whatever that is) would be a start - when combined with solutions to the other problems.