I fairly commonly find myself in a situation where I would like to display one label for certain features and another for other features in the same layer. QGIS doesn’t have an official way to split labels up into categories, and until now I’d resorted to having two layers to render otherwise identical features. But, in the new, excellent expression based labeling from Nathan Woodrow, I realized one can use an SQL CASE statement.

Data courtesy MapQuest and OpenStreetMap CC-BY-SA, the City and County of San Francisco, and Bay Area Rapid Transit This is an overlay of the transit and walking trip plans generated by OpenTripPlanner from Powell and Market to every other intersection in San Francisco, after Eric Fischer’s map of walking routes to every intersection in San Francisco. It brings out the transit routes but also shows well-used walking routes. The lines do not vary in width (don’t let Market Street fool you, it’s actually several lines—BART, MUNI rail in 2 directions, Muni bus, walking—very near each other).

I found this buried deep in an appendix of the Mapnik XML Schema Reference, and I thought it so useful I am reposting it here: Zoom level Scale denominator 0 559,082,264 1 279,541,132 2 139,770,566 3 69,885,283 4 34,942,642 5 17,471,321 6 8,735,660 7 4,367,830 8 2,183,915 9 1,091,958 10 545,979 11 272,989 12 136,495 13 68,247 14 34,124 15 17,062 16 8,531 17 4,265 18 2,133 19 1,066 20