Projection support wanted
Bjorn
TileMill is a great tool for creating nice looking interactive maps, but I would like alternatives to web mercator. Do you plan to add support for different projections in a future release?
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Support Staff 2 Posted by Dane on 02 Sep, 2011 01:11 AM
Bjorn,
Should we?
TileMill is certainly of the tribe of tools that target a single projection intentionally to try to make things simpler for a larger audience of non-specialist map makers.
That said, the underlying renderer, mapnik, is projection (and tile system) agnostic, so once you design your map in TileMill you can simply export out the Mapnik XML, setting the
srsto your desired projection, and render your maps from there (of course scale denominators may need to be adjusted).I think the larger issue is community standards around tiling schemes for projections. If it there were a clearer path to this I think there could be a discussion of TileMill supporting more projections, but I'm not seeing that at this point.
What do you think?
3 Posted by Bjorn on 02 Sep, 2011 06:14 AM
I got your point. The map maker would need to define a gridset for a spatial reference system à la GeoWebCache: http://geowebcache.org/docs/current/concepts/gridsets.html
Community standards around tiling schemes for projections is a great idea. Maybe you should start such an initiative! ;-)
As far as I understand, Mapnik renders images/grids and not tilesets. I've previously used TileCache together with Mapnik. Do you know of a tile script that supports multiple reference systems and the new grid renderer in Mapnik2?
Support Staff 4 Posted by Dane on 02 Sep, 2011 02:10 PM
"gridsets" would be a good thing to discuss at http://wiki.osgeo.org/wiki/FOSS4G_2011_Breakout_Sessions - are you perhaps attending?
The grid_renderer API in mapnik is very experimental at this point via the python API, but TileStache is planning support (https://github.com/migurski/TileStache/pull/12) and I think TileStache supports both 900913 and 4326 - so that is at least one more projection than TileMill :). The other option might be MapProxy.
5 Posted by Bjorn on 02 Sep, 2011 03:58 PM
Unfortunately, I'm not able to attend the FOSS4G conference this year (it look like it's going to be a great event!), but I hope you'll raise the issue at the breakout sessions.
Maybe it don't have to be that complicated for the map maker. I think you can standardize on 256x256px tiles and that each zoom level is dividing every tile into four - like you do with web mercator. You already have a nice interface for specifying map extent and zoom levels. It's everything you need to create tiles in a different projection. If the map maker use the same extent (and max resolution, which you can calculate on the fly) in OpenLayers or other capable map clients, the tiles should line up correctly.
It's a bit more complicated if the user want to mix projected tiles from different providers - as they should follow the same gridset pattern - but I'm sure you can make a nice interface for this task as well.
I think the json grids are a great contribution to the web mapping evolution (+ css-like styling and the beautiful Mapnik rendering), and I would love to use it for my thematic world maps and country maps. Mercator is a really bad choice, especially for a country far north like Norway. While waiting for projection support in TileMill, I'll have look at the tools your suggesting.
Support Staff 6 Posted by Dane on 02 Sep, 2011 07:39 PM
Thanks, glad you like the css and json advances. We do too:) The css styling and the json grids can be also used fully outside of tilemill - there is nothing specific about these to mercator. To leverage css you would use the
cartotoolkit (or wait for native carto support in mapnik: http://trac.mapnik.org/milestone/GSOC%20Carto%20C%2B%2B). And to leverage the json grids, as we've discussed, you would just use the mapnik python api or javascript api to render them in any projection.As far as getting the Mapnik XML styles from TileMill to use in another projection see: http://support.mapbox.com/kb/tilemill/exporting-mapnik-xml-from-til...
And yes, I think it could be doable for a mapping client, like OpenLayers, to support any projection, but we've move quite far from this in TileMIll specifically. We moved to using ModestMaps, which only talks in mercator, because we only planned support for mercator and ModestMaps is much smaller than OpenLayers.
So, I guess my summary is that I recognize the critical importance of using the right projection for a given map. My feeling is that TileMill, at least right now given the set of tools at hand, will benefit more users if we stick to one projection. But one projection for the map design stage only - TileMill obviously tries to help prepare you for the next stages (and guides you based on mercator) but this is not the only way and TileMill should not prevent more creative and custom uses.
Support Staff 7 Posted by Tom MacWright on 26 Oct, 2011 02:41 PM
Hi Bjorn,
If you want to propose a system for multiple projection support, please post to the TileJSON specification, MBTiles spec, and Carto spec with a proposal.
Generally we need a system that does the following:
Creating an interface for combining tilesets of different projections is a nonissue, since underlying goal is impossible without eliminating IE support or adding an expensive reprojection operation in the middle.
There are plenty of possibilities for solutions here, but nothing seems viable yet. In the middle-term, it seems possible to do static exports of specific projections so that the problem area is reduced tremendously. We're certainly open to good ideas, but it's important to understand that we're building for all users, not just advanced users, and we want the level of expectations that we've set - that MBTiles will just work, that maps can be autoconfigured and combined - to continue.
Closing this here: the general goal of projection support is agreed-upon, but it's only as good as its underlying architecture, which still needs creation.
Tom MacWright closed this discussion on 26 Oct, 2011 02:41 PM.