[OSM-talk] db and user statistics update

Dirk-Lüder Kreie osm-list at deelkar.net
Sat Jan 20 00:27:15 GMT 2007


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Christopher Schmidt schrieb:

> The WMS-C matches Google, which is the only 'right' aspect it has.
> Neither it nor the map which is based on a spheroid is 'right', you're
> absoltely correct.
> 
> However, matching other maps which are projected in simple mercator is
> an important use case for what I use the map for, so I won't be changing
> it.
> 
> I advise removing the WMS-C layer from the OSM homepage: it's confusing
> and useless in that interface.

The main problem being that the lat/lon <-> Tile relation is broken for
the WMS-C layer. Since both Tilesets (Mapnik and Mapnik WMS-C) originate
from the same dataset, the Map should not apparently jump kilometers
north/south on switching between those layers. Since the first layers
all display the correct area for any lat/lon coords I checked, the WMS-C
layer does not, which indicates something goes wrong when calculating
the tile numbers for the given latitude. This most probably has it's
source in the different projection, and fixing this will probably mean
the WMS-C tiles will not line up correctly if the applet forces the
projection, which I don't think it needs to do, because to the map all
tiles are flat squares, so the entire World is projected onto a flat
surface (actually cylindrical, but that doesn't change anything in this
case, other that there is no map border on 180° E or W).

The trick is choosing the right area to display from this flat map
surface, which is where the projection comes into play, especially on
the north-south axis, east-west doesn't matter if all tilesets use
powers of 2 for the total number of tiles around the equator.

Dirk-Lüder "Deelkar" Kreie




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