[postgis-users] a line' s direction

[Hartmut Tschauner]

Hi Ahmet,
In an earlier message that didn't arrive as part of this thread and with its
body scrapped, I posted a function (see below) that will calculate azimuth
between two points. You would pass the start and end points to get the
direction of a line.
Cheers,
Hartmut
Hartmut Tschauner
Department of Archaeology
Seoul National University
Gwanak-Gu Sillim 9-dong San 56-1
Seoul, 151-742
Korea
ph. +82 (2) 880-9260


CREATE OR REPLACE FUNCTION public.azimuth(geometry, geometry)
  RETURNS float8 AS
'
DECLARE
   geom1 ALIAS FOR $1;
   geom2 ALIAS FOR $2;
   geom2trans geometry;

BEGIN
   IF geom1 IS NULL OR geom2 IS NULL THEN
      RETURN NULL;
   ELSE
      IF isempty(geom1) OR isempty(geom2) OR geometrytype(geom1) !=
\'POINT\' OR geometrytype(geom2) != \'POINT\' THEN 
         RETURN NULL;
      END IF;
   END IF;

   IF srid(geom1) != -1 AND srid(geom2) != srid(geom1) THEN
      geom2trans := transform(geom2, srid(geom1));
   ELSE
      geom2trans := geom2;
   END IF;

   IF x(geom1) = x(geom2trans) AND y(geom1) < y(geom2trans) THEN 
      RETURN 0;
   ELSIF x(geom1) = x(geom2trans) AND y(geom1) > y(geom2trans) THEN 
      RETURN 180;
   ELSIF y(geom1) = y(geom2trans) AND x(geom1) < x(geom2trans) THEN 
      RETURN 90;
   ELSIF y(geom1) = y(geom2trans) AND x(geom1) > x(geom2trans) THEN 
      RETURN 270;
   ELSIF x(geom1) < x(geom2trans) AND y(geom1) < y(geom2trans) THEN 
      RETURN degrees(atan(abs(x(geom1) - x(geom2trans)) / abs(y(geom1) -
y(geom2trans))));
   ELSIF x(geom1) < x(geom2trans) AND y(geom1) > y(geom2trans) THEN 
      RETURN degrees(atan(abs(y(geom1) - y(geom2trans)) / abs(x(geom1) -
x(geom2trans)))) + 90;
   ELSIF x(geom1) > x(geom2trans) AND y(geom1) > y(geom2trans) THEN 
      RETURN degrees(atan(abs(x(geom1) - x(geom2trans)) / abs(y(geom1) -
y(geom2trans)))) + 180;
   ELSIF x(geom1) > x(geom2trans) AND y(geom1) < y(geom2trans) THEN 
      RETURN degrees(atan(abs(y(geom1) - y(geom2trans)) / abs(x(geom1) -
x(geom2trans)))) + 270;
   ELSE
      RETURN 0;
   END IF;

END;
'
  LANGUAGE 'plpgsql' VOLATILE;


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