[OSM-talk] gdalwarp question

Juan Lucas Dominguez Rubio jldominguez at prodevelop.es
Sun May 3 16:14:07 BST 2009


Please do not mislead people. gdalwarp works fine:
 
http://tinyurl.com/germany-blue-marble
 
Regards
Juan Lucas
 
 

		 

________________________________


		 

		Hello Jukka,
		
		thanks for your hint, but to my understanding this contradicts
		the description of RasterImage in Mapnik and the web page
		that converts "blue marble" using gdalwarp.
		
		In the documentation for RasterImage it says that Mapnik
		can't (yet) reproject reaster image data, they have to match
		the target projection.  I got that answer to another question:
		
		http://www.mail-archive.com/mapnik-users@lists.berlios.de/msg01213.html
		
		
		So if in osm.xml there is:
		
		srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0
		+x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs +over"
		
		Then i think this should be the target projection (if i want to mix a raster
		image into it).
		
		
		The "blue marble" has lat / lon directly mapped to x / y coordinates.
		To my understanding this is similar to WGS84, to add the scaling
		with a "world file" makes sense to me.
		
		Looking at this page  i see a description of projections and the use of
		gdalwarp that describes how to get
		
		http://egb13.net/2009/04/bending-the-earth-gdalwarp-and-the-blue-marble/
		
		
		But in the end, my understanding did not leave to a correctly projected
		image...
		
		
		Best regards,
		Torsten.
		
		
		Am Sonntag, 3. Mai 2009 14:11:06 schrieb Jukka Rahkonen:
		> Torsten Mohr <tmohr <at> s.netic.de> writes:
		> > Hello,
		> >
		> > thanks for that hint.
		> >
		> > Right, the chosen projection won't work around the poles, i don't expect
		> > that.  In the call to gdalwarp i gave the source projection (EPSG:4326 or
		> > WGS84) and as target projection i gave the projection used in osm.xml.
		>
		> Hi,
		>
		> Your target projection is the so called Google projection, or epsg:900913,
		> or nowadays officially epsg:3785.  It is the projection used in OSM slippy
		> map, but the native OSM data are in epsg:4326.  Therefore you should warp
		> the downloaded images _into_ epsg:4326, not from that.  Unfortunately I
		> cannot say what would be the correct source projection definition for your
		> original images.
		>
		> An easy way to test your warped images is to download some OSM data in
		> shapefile format and in epsg:4326 projection from Geofabrik.de, open the
		> warped image with some GIS program like QGis or OpenJUMP together with OSM
		> shapefile and see if they suit well together.
		>
		> Gdalwarp options are documented at http://gdal.org/gdalwarp.html but some
		> further reading may be necesssary to understand what all the options mean.
		>
		> -Jukka Rahkonen-
		>
		>
		>
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