Name

ST_LineCrossingDirection — Returns a number indicating the crossing behavior of two LineStrings

Synopsis

`integer ST_LineCrossingDirection(`geometry linestringA, geometry linestringB`)`;

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 |         1
```

Example: 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 |        -3
```

Example: 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 |         2
```

Example: 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;
```