[postgis-users] Calculate the distance between a point and a line

[Luca Bertoncello]

Hi list!

I'm new here and I use Postgis with OpenStreetMap data to render maps in 
ICAO-style for using in a App I'm currently develop.

My problem: I have a Postgis-Database with the OpenStreetMap-data for 
Europe.
 From other source (Eurocontrol) I have a list of reporting points, some 
of them will be used by hobby pilots, too. These are reporting points at 
(or near) the country boundaries.
And I want to get a list of these (and just these!) point.

Let's assume, the points are max 20 Km from the boundary, I tried to 
calculate the distance between a point (OMELO, coordinates 
50.8379138888889 14.0007583333333) and the boundaries:

SELECT
   name,
   admin_level,
   ST_Distance(ST_Transform('SRID=4326;POINT(50.8379138888889 
14.0007583333333)'::geometry, 3857), way) * cosd(42.3521) as dist
FROM planet_osm_roads
WHERE boundary = 'administrative'
   AND admin_level = '2'
   AND osm_id < 0
ORDER BY admin_level DESC;

Unfortunately I only get values that make no sense, since OMELO is a 
couple of Km to the Czech Republic, but I get:

     name     | admin_level |       dist
-------------+-------------+------------------
Česko       | 2           | 4812054.73000158
Deutschland | 2           | 4812054.73000158
Česko       | 2           |  4812014.2751998
Deutschland | 2           |  4812014.2751998
Česko       | 2           | 4801322.76686319
Deutschland | 2           | 4801322.76686319
Česko       | 2           | 4798301.61335419
Deutschland | 2           | 4793808.13783476
Česko       | 2           | 4786576.73402148
Deutschland | 2           | 4785266.87451821
Česko       | 2           | 4775493.77594454
Deutschland | 2           | 4775493.77594454
Česko       | 2           | 4746495.15621187
Deutschland | 2           | 4746495.15621187
Česko       | 2           | 4775724.71111992
Deutschland | 2           | 4767387.92338493
Polska      | 2           | 4748955.14857684
Polska      | 2           |  4759696.4876026
Deutschland | 2           |  4759696.4876026
Deutschland | 2           |  4849836.9269982
Polska      | 2           |  4849836.9269982
Polska      | 2           | 4797231.65721602
Deutschland | 2           | 4797231.65721602
Polska      | 2           | 4890476.28917314
Deutschland | 2           | 4890476.28917314
Polska      | 2           | 4938293.09438962
Deutschland | 2           | 4938293.09438962
Deutschland | 2           | 5002527.32842157
Polska      | 2           | 5002527.32842157
(29 Zeilen)

Can someone help me?

Thank you very very much!
Luca Bertoncello
(lucabert at lucabert.de)