Clean up low-zoom levels

[matthijsmelissen]

• Render roads from z6 instead of z5
• Render national parks from z8 instead of z7
• Render railways from z8 instead of z7

This is in line with the discussion on use cases and USECASES.md.

This contributes towards #2925 and #2688.

[matthijsmelissen]

@kocio-pl As you are also working on the low-level stuff, what do you think of this PR?

[kocio-pl]

Sure, I plan to do it soon, just trying to collect all the pieces in my head... 😄

[matkoniecz]

I like the idea of this changes.

[rrzefox]

Preview of this (and other pending changes) is available at http://bl.ocks.org/math1985/raw/af7a602c222dbf1ff1a2c0d84ed755b7/

[kocio-pl]

Preview helped me a lot, thanks for it!

I approve all the changes:

1. The most important is moving roads later (resolves new road colours lead to roads dominating z5 #1935 BTW) - half of the USA, China and Europe, the whole Japan, India and Turkey are now more readable. On z6 they are not too dense anywhere.
2. Nature reserves are currently used more or less as a placeholder for natural areas, but they have too many borders (maybe they should be thinner anyway, even on z8+), and lack the natural context.
3. I was not aware how many railroads are visible on z7 and that there are also a lot of smaller, local lines, not just main ones (like here).

[matkoniecz]

Preview of this (and other pending changes) is available

Thanks, this is a great help! Btw, what is the best way to report issues? This page send to be not working on mobiles - attempting to zoom out/in results in moving to NaN/NaN coordinates.

[matthijsmelissen]

Thanks for reviewing!