PostGIS Line Intersection – How to Find Intersection Between Two Line Segments

intersectionlinepointpostgis

I am trying to find the intersection point of two lines of which I know 2 points.

For example, I have these 2 line segments:

select st_geometryfromtext('LINESTRING (4.50554487780521 51.922119943633575, 4.504656820078167 51.92231795217855)', 4326),
st_geometryfromtext('LINESTRING (4.505476754241158 51.92221504789901, 4.505379267847784 51.92221833721103)', 4326)

As it can be seen on the map,they do not have an intersection point:

What I am interested in finding is the intersection point of the lines which these segments belong to, as shown below:

Best Answer

Just extend (extrapolate) one of your lines and get the intersection point as:

select st_intersection((select st_geometryfromtext('LINESTRING (4.50554487780521 51.922119943633575, 4.504656820078167 51.92231795217855)', 4326)),
(select ST_MakeLine(ST_TRANSLATE(a, sin(az1) * len, cos(az1) * 
len),ST_TRANSLATE(b,sin(az2) * len, cos(az2) * len))
  FROM (
    SELECT a, b, ST_Azimuth(a,b) AS az1, ST_Azimuth(b, a) AS az2, ST_Distance(a,b) + 1 AS len
      FROM (
        SELECT ST_StartPoint(st_geometryfromtext('LINESTRING (4.505476754241158 51.92221504789901, 4.505379267847784 51.92221833721103)', 4326)) AS a, 
        ST_EndPoint(st_geometryfromtext('LINESTRING (4.505476754241158 51.92221504789901, 4.505379267847784 51.92221833721103)', 4326)) AS b
          FROM ST_MakeLine(ST_MakePoint(1,2), ST_MakePoint(3,4)) AS the_geom
    ) AS sub
) AS sub2));

extrapolation example from How to extend a straight line in postgis?

Related Solutions

PostGIS Intersect – How to Identify Intersecting Line Segments in PostGIS

To find lines which intersect in the interior of both lines, a query is:

CREATE TABLE intersection_points as
SELECT      
    ST_Intersection(a.geom, b.geom) AS intpt
FROM a 
JOIN b ON ST_Intersects(a.geom, b.geom)
WHERE a.id < b.id 
      AND NOT ST_Touches(a.geom, b.geom);

To find intersections where at most one of the lines can intersect at an endpoint, the query has to select line pairs which intersect but where their boundaries do not intersect (the DE-9IM pattern F):

CREATE TABLE intersection_points as
SELECT      
    ST_Intersection(a.geom, b.geom) AS intpt
FROM a 
JOIN b ON ST_Intersects(a.geom, b.geom)
WHERE a.id < b.id 
      AND ST_Relate(a.geom, b.geom, '****F****');

Best Answer

Related Solutions

PostGIS Intersect – How to Identify Intersecting Line Segments in PostGIS

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