[postgis-users] Help: Reverse Geocoding with OSM data

[Brian Modra]

2009/9/26 Rickus <rickus at mnt-direct.com>:
> Hi
> I hope someone can put me on the right track. I'm new to this list and new
> to PostGis and to Spatial Databases.
>
> I want to do Reverse Geocoding on OpenStreetMap.org data. The requirement
> is:
> Given a lat/long the query needs to return the closest road and point and
> suburb, city etc. The biggest requirement is that it should be very fast.
>
> I have a PgSQL/PostGis DB Server running with a subset of the OpenStreetMap
> database. In the OSM DB there are tables with 'planet_osm_point',
> 'planet_osm_roads', and 'planet_osm_line' data. I also added a 900913
> mercator projection to the 'spatial_ref_sys' table.
>
> I have been exploring queries similar to this one:
> "Select name FROM planet_osm_line WHERE distance (way,
> geomfromtext('POINT(-9393780.6113322 3993956.8318506)', 900913)) < 200;"
>
> But I guess where I get lost is that:
> - I need to search in all the relevant tables for line, point and roads?
> - I need to sort according to distance from the given lat/long to get the
> closest point?
> - I need to use a 'bounding box' but not sure how to do it. I guess a
> bounding box is needed to limit the search to a restricted area?

Your SQL will do a sequential read of the entire table, calculating
the distances of each.
What you need to do is:
1) narrow the search using the spatial index
2) then get the one thats closest
You need to use plpgsql.

e.g. I'm assuming the table has a unique ID for each row, called "ID",
replace this with the actual ID, and check that your table has an
index on that column.
I'm also assuming your table has a "way" geometry column as you used
in your example above:

create or replace function getClosest(
 lat double precision,
 lon double precision,
 tol double precision)
returns text as $$
declare
 point geometry;
 rec record;
begin
 point := geomfromtext('POINT('||lon||' '||lon||')', 900913);
 select id, distance(way, point) < tol as dist
 into rec
 from planet_osm_line where way && point
 order by dist asc limit 1;
 return rec.name;
end;
$$ language plpgsql;

Please excuse me if this does not (quite) work, I have not tried it...
only typed it into this email rather quuickly. But in any case, it
should put you on the right track.
> Any help will much be appreciated!
> Rickus
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>



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