Calculate localrank/scalerank

[tsauerwein]

See labelgrid: https://github.com/mapbox/postgis-vt-util

https://www.mapbox.com/developers/vector-tiles/mapbox-streets

Controlling label density

The localrank field can be used to adjust the label density by showing fewer labels. This method is preferred to CartoCSS’s text-min-distance because it leads to far fewer labels clipped across tile boundaries. It is a whole number >=1 calculated by grouping places into a ~300m projected grid, then assigning each place a ranking within that grid. The most important place in each cell will get a localrank of 1, the second most important is 2, and so on.

Scalerank

The scalerank field can be used to adjust the prominence of label styles for larger and more prominent cities. The value number from 0 through 9, where 0 is the large end of the scale (eg New York City). All places other than large cities will have a scalerank of null.