[OSM-talk] find corners of exported maps

Olav Kvittem Olav.Kvittem at uninett.no
Thu Dec 11 10:32:51 GMT 2008



Dirk-Lüder Kreie wrote:
> have a look at the debugHelper.pl in tiles at home. it will give you the
> latlon center of the tile bbox and the latlon of the center of the tile.

Thanks, but I still don't get it and need some more hints.
This is about finding the corners of a tile which is a discrete unit
with discrete center points.
The tahproject.pm computes the mercator tile height the way I do and
Tileset.pm also uses 256 pixel tile image size.
I am guessing that the osmarender export function  deliver tile density
  for any centerpoint, so I probably need to reach to the guy
producing the export code or looking at the code itself to know how to
compute corner coordinates for this : any center, any size "tile".

The exporter takes the geodetic center as a parameter, but I
expect it to deliver a map evenly around the center of the projection.
If not I don't see how I can find the corners.

chuss
Olav

> Olav Kvittem schrieb:
>> Hi,
>>
>> I wonder how I compute the coordinates of the corners of the images 
>> received from the web export function
>> for an osmarender image. I specify geodetic center position(degrees), 
>> zoom level and image size and get at nice map image.
>> The projection is of cource Mercator and I can linearly map mercator 
>> coordinates onto the image.
>> But how do I know the coordinates of the corners of the returned image ?
>>
>> The url I use is :
>> http://server.tah.openstreetmap.org/MapOf/index.php?long=10.17456&lat=56.01566&z=6&w=500&h=592&format=png' 
>>
>>
>> The tile-size for each level is 256x256 so given the zoom-level I could 
>> compute the pixel density from the
>> world map size in mercator projection.
>> That works for east direction which is linear, but for the northern 
>> direction the projection is nonlinear and
>> I am not sure if the geodetic center that I specify is at the center of 
>> the returned image or not.
> 
> have a look at the debugHelper.pl in tiles at home. it will give you the
> latlon center of the tile bbox and the latlon of the center of the tile.
> 
> 
> 
> 
> 
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