[OSM-dev] osm2pgsql - coordinates units

[Jon Burgess]

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

The factor of 100 is a feature and is setup at [1]. It allows
osm2pgsql to store the position as a fixed point 32 it integer taking
just 4 bytes, instead of the 8 bytes required for a double precision
floating point value.

osm2pgsql uses the lat/lon fields when looking up the node locations.


1: http://trac.openstreetmap.org/browser/applications/utils/export/osm2pgsql/middle-pgsql.c#L27

-- 
    Jon