PostGIS
Name
ST_LineCrossingDirection — Returns a number indicating the crossing behavior of two LineStrings
Description
Given two linestrings returns an integer between -3 and 3 indicating what kind of crossing behavior exists between them. 0 indicates no crossing. This is only supported for `LINESTRING`s.
The crossing number has the following meaning:
-
0: LINE NO CROSS
-
-1: LINE CROSS LEFT
-
1: LINE CROSS RIGHT
-
-2: LINE MULTICROSS END LEFT
-
2: LINE MULTICROSS END RIGHT
-
-3: LINE MULTICROSS END SAME FIRST LEFT
-
3: LINE MULTICROSS END SAME FIRST RIGHT
Availability: 1.4
Examples
Example: LINE CROSS LEFT and LINE CROSS RIGHT
Blue: Line A; Green: Line B
SELECT ST_LineCrossingDirection(lineA, lineB) As A_cross_B,
ST_LineCrossingDirection(lineB, lineA) As B_cross_A
FROM (SELECT
ST_GeomFromText('LINESTRING(25 169,89 114,40 70,86 43)') As lineA,
ST_GeomFromText('LINESTRING (20 140, 71 74, 161 53)') As lineB
) As foo;
A_cross_B | B_cross_A
-----------+-----------
-1 | 1Example: LINE MULTICROSS END SAME FIRST LEFT and LINE MULTICROSS END SAME FIRST RIGHT
Blue: Line A; Green: Line B
SELECT ST_LineCrossingDirection(lineA, lineB) As A_cross_B,
ST_LineCrossingDirection(lineB, lineA) As B_cross_A
FROM (SELECT
ST_GeomFromText('LINESTRING(25 169,89 114,40 70,86 43)') As lineA,
ST_GeomFromText('LINESTRING(171 154,20 140,71 74,161 53)') As lineB
) As foo;
A_cross_B | B_cross_A
-----------+-----------
3 | -3Example: LINE MULTICROSS END LEFT and LINE MULTICROSS END RIGHT
Blue: Line A; Green: Line B
SELECT ST_LineCrossingDirection(lineA, lineB) As A_cross_B,
ST_LineCrossingDirection(lineB, lineA) As B_cross_A
FROM (SELECT
ST_GeomFromText('LINESTRING(25 169,89 114,40 70,86 43)') As lineA,
ST_GeomFromText('LINESTRING(5 90, 71 74, 20 140, 171 154)') As lineB
) As foo;
A_cross_B | B_cross_A
-----------+-----------
-2 | 2Example: Finds all streets that cross
SELECT s1.gid, s2.gid, ST_LineCrossingDirection(s1.geom, s2.geom)
FROM streets s1 CROSS JOIN streets s2
ON (s1.gid != s2.gid AND s1.geom && s2.geom )
WHERE ST_LineCrossingDirection(s1.geom, s2.geom) > 0;