[OSM-dev] OpenStreetMap Cartographic: A client-side rendered OpenStreetMap Carto

[Paul Norman]

I've been working on a new project, OpenStreetMap Cartographic. This is
a client-side rendering based on OpenStreetMap Carto. This is an
ambitious project, as OpenStreetMap Carto is an extremely complex style
which shows a large number of features. The technical choices I'm making
are designed so the style is capable of handling the load of osm.org
with minutely updates.

I've put up a world-wide demo at 
https://pnorman.dev.openstreetmap.org/cartographic/mapbox-gl.html,
using data from 2020-03-16, and you can view the code at 
https://github.com/pnorman/openstreetmap-cartographic.

Incomplete parts
================

Only zoom 0 to 8 has been implemented so far. I started at zoom 0 and am
working my way down.

Admin boundaries are not implemented. OpenStreetMap Carto uses
Mapnik-specific tricks to deduplicate the rendering of these. I know how
I can do this, but it requires the changes I intend to make with the flex
backend.

Landuse, vegetation, and other natural features are not rendered until
zoom 7. This is the scale of OpenStreetMap Carto zoom 8, and these
features first appear at zoom 5. There are numerous problems with
unprocessed OpenStreetMap data at these scales. OpenStreetMap Carto gets
a result that looks acceptable but is poor at conveying information by
tweaking Mapnik image rasterizing options. I'm looking for better options
here involving preprocessed data, but haven't found any.

I'm still investigating how to best distribute sprites.

Technology
==========

The technology choices are designed to be suitable for a replacement for
tile.osm.org. This means minutely updates, high traffic, high reliability,
and multiple servers. Tilekiln, the vector tile generator, supports all
of these. It's designed to better share the rendering results among
multiple servers, a significant flaw with renderd + mod_tile and the
standard filesystem storage. It uses PostGIS' ST_AsMVT, which is very
fast with PostGIS 3.0. On my home system it generates z0-z8 in under
40 minutes.

Often forgotten is the development requirements. The style needs to
support multiple developers working on similar areas, git merge conflicts
while maintaining an easy development workflow. I'm still figuring this
out. Mapbox GL styles are written in JSON and most of the tools
overwrite any formatting. This means there's no way to add comments to
lines of codes. Comments are a requirement for a style like this, so
I'm investigating minimal pre-processing options. The downside to this
will make it harder to use with existing GUI editors like Fresco or 
Maputnik.

Cartography
===========

The goal of this project isn't to do big cartography changes yet, but
client-side rendering opens up new tools. The biggest immediate change
is zoom is continuous, no longer an integer or fixed value. This means
parameters like sizes can smoothly change as you zoom in and out,
specified by their start and end size instead of having to specify each
zoom.

Want to help?
=============

Have a look at https://github.com/pnorman/openstreetmap-cartographic and
have a go at setting it up and generating your own map. If you have
issues, open an issue or pull request. Or, because OpenStreetMap
Cartographic uses Tilekiln have a look at 
https://github.com/pnorman/tilekiln/issues.