[OSM-dev] Migrating osm.org to vectors/Kartotherian

[Christoph Hormann]

On Saturday 31 October 2015, Yuri Astrakhan wrote:
>
> Christoph, re multiple projections - if Mapnik implements vector tile
> merging, we could in theory dynamicaly recombine tiles and change
> projections, would't we? At a performance cost of course.

First of all if you don't use the tiles as tiles but merge them into a 
full data set again there is no point in generating them in the first 
place of course.

Apart from that regarding the abstract possibility: probably no for a 
number of reasons:

- since mercator tiles don't include areas near the poles you will not 
be able to handle any projections that include the poles although 
better display of polar regions is of course one of the primary reasons 
for using other projections.
- in general the tile edges in mercator will not be straight lines in 
other projections.  You would have to specifically take this into 
account when generating and processing the tiled data, otherwise you 
can't reproject the tiles without generating artefacts or invalid 
geometries.
- you cannot map the zoom levels in one projection to those in another, 
especially not with the strongly variable scale of the mercator 
projection so unless your tiles are not zoom level specific you are 
screwed.

These points apply to all generic geometry data in the tiles, any data 
specifically generated for rendering like label positions etc. is 
normally projection specific anyway.

-- 
Christoph Hormann
http://www.imagico.de/