[OSM-talk] osm2pgsql and proper/legacy mercator

[Juan Lucas Dominguez Rubio]

Hi, according to the source code, the legacy option -M is using this projection:
 
  [PROJ_MERC]    = { 
     descr: "WGS84 Mercator", 
     proj4text: "+proj=merc +datum=WGS84  +k=1.0 +units=m +over +no_defs", 
     srs:3395, 
     option: "" }
 
Unlike EPSG:900913, that projection is not assuming that the Earth is a perfect sphere, therefore:
 
- it seems to be cartographically more correct
- it probably needs far more computation
- the result probably differs from that of EPSG:900913 by a few meters
 
Regards
Juan Lucas
 
 

________________________________

		De: talk-bounces at openstreetmap.org en nombre de Francois Van Der Biest
		Enviado el: vie 01/05/2009 17:52
		Para: talk at openstreetmap.org
		Asunto: [OSM-talk] osm2pgsql and proper/legacy mercator
		
		

		Hi list,
		
		osm2pgsql --help says:
		-m|--merc: Store data in proper spherical mercator (default)
		-M|--oldmerc: Store data in the legacy OSM mercator format
		
		I'm wondering what's the difference between those two srs.
		Which one is epsg:900913 (aka epsg:3785
		http://www.spatialreference.org/ref/epsg/3785/) ?
		
		My experience (importing an osm dump into postgis, then exporting to
		shapefiles) would let me think that the "legacy OSM mercator format"
		is epsg:3785. So, what's the other one ?
		
		Thank's,
		F.
		
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