pgmapcss - Advanced Cartography for Mapnik
Implementing MapCSS by Moving the Cartography Into the Database
pgmapcss combines MapCSS - a more or less standardized map description language - with Mapnik - a widely used map renderer for vector or bitmap maps. In contrast to other attempts at using CSS-like styling, the actual cartography process (evaluating which map features should be rendered in the current view and how) is moved into a database function (Using PL/Python3 in a PostgreSQL database). Mapnik just needs to read the final properties - e.g. geometry, widths, colors, texts, and so on. This database function is compiled by pgmapcss from the MapCSS style sheet.
There are many advantages to this idea:
- Writing MapCSS simplifies writing style sheets for Mapnik, as there's no longer a separation into database query and styling. Often those database queries were complex and required advanced knowledge of SQL. pgmapcss takes most of the complexity away. In addition, the actual database queries for map features are optimized for each zoom level.
- MapCSS is powerful because map features can be related to each other (e.g. by relation membership or the proximity of objects). Furthermore calculations are possible (e.g. show the width of a line based on the value of a tag of the map feature). pgmapcss even offers geometric calculations (e.g. create lines between map features, rotation, buffers, …).
- As there are more than ten libraries which implement MapCSS, the language is available on different platforms and for different output media. Styles can be used with various libraries, although there are differences between dialects, but hopefully they will converge in the future.