[OSM-dev] osm2pgsql - coordinates units

[Graham Jones]

I think I have found the problem - I was using planet_osm_nodes lat and lon
fields.  If I use the planet_osm_point way field, and extract the
coordinates using ST_X() and ST_Y() all of the numbers match up.

You can see the factor of 100 difference if you do:
      select id,lat,lon,astext(way) from planet_osm_nodes, planet_osm_point
where id=osm_id and id=27496146;

This gives:
   mapnik=# select id,lon,lat,astext(way) from planet_osm_nodes,
planet_osm_point where id=osm_id and id=27496146;
       id    |    lon    |    lat    |
astext

----------+-----------+-----------+-------------------------------------------
    27496146 | -13446488 | 730126793 | POINT(-134464.883471724
7301267.93936479)
   (1 row)

   mapnik=#

Is this a bug in osm2pgsql, or a 'feature'?   Does anything use the lat/lon
fields?

Graham.

2010/1/2 Graham Jones <grahamjones139 at googlemail.com>

> Hi,
> I have a little Python application that populates a posgresql database
> using osm2pgsql, renders a map with mapnik and extracts information from the
> database to highlight points of interest.
> I am having a bit of trouble with working out what units the coordinates of
> nodes are in in the postgresql database and hope it is obvious to someone:
>
> I am using the -m option, which I think means it will use a spherical
> mercator projection ("+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").  I am using the same projection in my Python code.
>
> What I find though is that when I use the mapnik.Projection() function to
> translate from lat/lon to spherical mercator y/x, my calculated values are a
> factor of 100 less than those in the postgresql database - If I just divide
> the postgresql database values by 100 and I plot them on my map they end up
> in the correct places.
>
> It looks to me as though osm2pgsql must be storing the x/y values as
> integers in units of centimetres, but I thought that it was supposed to be
> in units of metres.
>
> Is this the case, or have I got something wrong with my projections (If the
> answer is not obvious I will go and work out the maths myself to see what
> order of magnitude the xy values should be - I am not used to working in
> these coordinates).
>
> Thanks,
>
>
> Graham.
>
> --
> Dr. Graham Jones
> Hartlepool, UK
> email: grahamjones139 at gmail.com
>



-- 
Dr. Graham Jones
Hartlepool, UK
email: grahamjones139 at gmail.com
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