[OSM-dev] Convert from Spherical Mercator to WGS-84?

[Dane Springmeyer]

On Nov 28, 2009, at 5:58 PM, Mike N. wrote:
>
>  I have definitely not taken the GIS:100 course!  I could not  
> manipulate
> the epsg library arguments into cs2cs argments, but I found a Python
> fragment that does what I need, so I'm set.
>
> ....
> self.originShift = 2 * math.pi * 6378137 / 2.0
> ...
> def MetersToLatLon(self, mx, my ):
>        "Converts XY point from Spherical Mercator EPSG:900913 to lat/ 
> lon in
> WGS84 Datum"
>
>        lon = (mx / self.originShift) * 180.0
>        lat = (my / self.originShift) * 180.0
>
>        lat = 180 / math.pi * (2 * math.atan( math.exp( lat * math.pi /
> 180.0)) - math.pi / 2.0)
>        return lat, lon
>
>
> -------------- Reverse with cs2cs:
>
> echo -180 45 | cs2cs +proj=latlong +datum=WGS84 +to +proj=merc  
> +ellps=sphere
> +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0  
> +units=m
> +towgs84=0,0,0 +no_defs
>
>

The MetersToLatLon looks good, but careful with the cs2cs command, as  
that proj4 string for merc looks like the a variation on spherical  
mercator (as there are several).

More python examples below if you are into python (the first link  
includes the proj4 string for mercator that will match the  
MetersToLatLon result).

Dane

http://mapnik-utils.googlecode.com/svn/example_code/longlat_merc_conversion/
http://mapnik-utils.googlecode.com/svn/sandbox/tools/nikproj/nikproj.py
http://mapnik-utils.googlecode.com/svn/example_code/proj_transform/

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstreetmap.org/pipermail/dev/attachments/20091128/125f79bc/attachment.html>