[OSM-dev] Projection Issues

[Christopher Schmidt]

On Wed, May 31, 2006 at 07:08:28PM +0100, Nick Whitelegg wrote:
> Without an in-depth knowledge of projections, I know that tile.js takes in
> a "real" longitude and latitude and converts the latitude to a Mercator
> latitude (see e.g. http://nick.dev.openstreetmap.org/javascript/tile.js)
> The OSM tile renderer then accepts a bounding box of regular longitudes
> and Mercator latitudes.
> 
> Actually I don't think that's quite right... it converts the regular 
> latitudes into Mercator latitudes (e.g. 51N becomes 59N) in order to 
> divide up the tiles evenly. However the tile server itself accepts regular 
> latitudes, i.e. if you ask it for 51N, 1W it will give you data actually 
> at 51N, 1W. This is evidenced by accessing the OSM tile server directly:
> 
> 
> http://tile.openstreetmap.org/cgi-bin/steve/mapserv?map=/usr/lib/cgi-bin/steve/wms.map&service=WMS&WMTVER=1.0.0&REQUEST=map&BBOX=-1.5,50.8,-1.3,51.0&WIDTH=400&HEIGHT=320&LAYERS=landsat,streets
> 
> The Landsat you get is recognisable as the Southampton area which is at 
> that latitude and longitude.

Right -- the landsat imagery comes back correctly. But the Streets layer
doesn't: see http://crschmidt.net/projects/openstreetmap/openlayers
which calls out directly to the OSM WMS for tiles, and you'll see what I
mean.

-- 
Christopher Schmidt
Web Developer