[OSM-talk] project

[Jon Burgess]

On Sun, 2010-04-11 at 22:36 +0200, Torsten Mohr wrote:
> Hello,
> 
> i created a GIS database and imported the planet data with osm2pgsql -m.
> So the data is stored in mercaator format.
> 
> When executing this raw SQL query:
> 
> select st_X(way), st_Y(way), name from planet_osm_point where capital='yes';
> 
> Then i get some data like:
> 
>        st_x        |       st_y        |            name
> -------------------+-------------------+----------------------------
>   -18915583.203791 | -2162182.86527014 | Alofi
>  -11035458.4801666 |  2205926.10968281 | Ciudad de México
>  -10075876.3988655 |  1648035.49949994 | Guatemala City
> 
> How can i reproject the st_x and st_y to latitude / longitude?
> 
> I'd like to do this in an own program.  I already reprojected some
> Tiff data from latitude / longitude to mercaator, that worked fine.
> 
> But i don't know what st_x and st_y actually are, how they are scaled,
> what is their offset.  The documentation for st_X and st_Y didn't help
> me for this issue.
> 
> Can anybody please explain to me what st_x and st_y are, how they are scaled,
> what is their offset so i can reproject these data to latitude / longitude?

The are coordinates are plain 900913 data with no scaling or offset.
Postgis can project this back to latlong (4326) for you, e.g.

select st_X(wayLL), st_Y(wayLL), name from (select ST_AsText(ST_Transform(way,4326)) as wayLL, name from planet_osm_point where capital='yes') as foo limit 5;  
    st_x    |       st_y        |       name        
------------+-------------------+-------------------
 15.0489992 |        12.1130018 | N’Djamena نجامينا
 15.3125301 |        -4.3102408 | 
  9.4540009 | 0.390002200000005 | Libreville
 10.1862628 |        36.8002392 | تونس
 14.4212127 |        50.0876559 | Praha
(5 rows)

Alternatively there are formulas on the wiki to convert between latlong
and 900913. http://wiki.openstreetmap.org/wiki/Mercator

	Jon