Lines

Find Lines which have a given angle of incidence
1. See Also
Find lines which are covered by a Line using a tolerance
Find Line Intersections at endpoints
Find Line Intersections NOT at endpoints
Count number of intersections between Line Segments
Find Lines which intersect N Polygons
1. Solution
Find Begin and End of circular sublines
Find Lines that form Rings
Find Longest Line Segment
Find non-monotonic Z ordinates in a LineString
Query LineString Vertices by Measures
Query Invalid Zero-Length LineStrings

Find Lines which have a given angle of incidence

https://gis.stackexchange.com/questions/134244/query-road-shape

Find lines which are covered by a Line using a tolerance

https://gis.stackexchange.com/questions/408581/overlapping-lines-on-vector

Solution Use ST_Snap:

WITH lines AS (SELECT (ST_Dump(ST_GeomFromText(
    'GEOMETRYCOLLECTION(
        LINESTRING(-76.4631041 38.9533412, -76.45514403057082 38.962161103032216),
        LINESTRING(-76.4631041 38.9533412, -76.45349643519081 38.96398666895439),
        LINESTRING(-76.45349643519081 38.96398666895439,-76.4525346 38.9650524),
        LINESTRING(-76.45514403057082 38.962161103032216,-76.45349643519081 38.96398666895439)
    )'
))).geom AS geom)
SELECT ST_Covers(a.geom, ST_Snap(b.geom, a.geom, 0.01))
FROM lines a, lines b;

Find Line Intersections at endpoints

https://gis.stackexchange.com/questions/20835/identifying-road-intersections-using-postgis

SELECT a.id AS aid, b.id AS bid,
    ST_Force2D( ST_Intersection(a.geom, b.geom)) AS geom 
FROM paths AS a 
JOIN paths AS b ON ST_Intersects(a.geom, b.geom) 
WHERE a.id < b.id AND ST_Touches(a.geom, b.geom);

Find Line Intersections NOT at endpoints

https://gis.stackexchange.com/questions/431246/in-postgis-how-to-select-intersecting-line-segments-but-not-matching-start-and https://gis.stackexchange.com/questions/433140/detecting-overshoot-lines-in-road-network-flyovers-over-underpasses-in-postgi/433202#433202

Solution Query lines which intersect but which do not just touch (at endpoints).

SELECT a.id AS aid, b.id AS bid,
    ST_Force2D( ST_Intersection(a.geom, b.geom)) AS geom 
FROM paths AS a 
JOIN paths AS b ON ST_Intersects(a.geom, b.geom) 
WHERE a.id < b.id AND NOT ST_Touches(a.geom, b.geom);

Count number of intersections between Line Segments

https://gis.stackexchange.com/questions/365575/counting-geometry-intersections-between-two-linestrings

Find Lines which intersect N Polygons

https://gis.stackexchange.com/questions/349994/st-intersects-with-multiple-geometries

Solution

Nice application of counting using HAVING clause.

SELECT lines.id, lines.geom 
FROM lines
 JOIN polygons ON st_intersects(lines.geom,polygons.geom)
WHERE polygons.id in (1,2)
GROUP BY lines.id, lines.geom 
HAVING count(*) = 2;

Find Begin and End of circular sublines

https://gis.stackexchange.com/questions/206815/seeking-algorithm-to-detect-circling-and-beginning-and-end-of-circle

Find Lines that form Rings

https://gis.stackexchange.com/questions/32224/select-the-lines-that-form-a-ring-in-postgis Solutions Polygonize all lines, then identify lines which intersect each polygon Complicated recursive solution using ST_LineMerge!

Find Longest Line Segment

https://gis.stackexchange.com/questions/359825/get-the-maximum-distance-between-two-consecutive-points-in-a-linestring

Find non-monotonic Z ordinates in a LineString

https://gis.stackexchange.com/questions/374459/postgis-validate-the-z-continuity

Assuming the LineStrings are digitized in the correct order (start point is most elevated), this returns all vertices geom, their respective line , and their position in the vertices array of that line, for all vertices with higher Z value as their predecessor.

SELECT ln_id, vtx_id, geom
FROM (SELECT ln.<id> AS ln_id,
         dmp.path[1] AS vtx_id,
         dmp.geom,
         ST_Z(dmp.geom) < LAG(ST_Z(dmp.geom)) OVER(PARTITION BY ln.<id> ORDER BY dmp.path[1]) AS is_valid
  FROM   <lines> AS ln,
         LATERAL ST_DumpPoints(ln.geom) AS dmp
) q
WHERE NOT is_valid;

Query LineString Vertices by Measures

https://gis.stackexchange.com/questions/340689/measuring-4d-relation-querying-within-linestring-using-postgis

Given a Linestring wit timestamps as measure value, find segments where duration between start and end timestamps is larger than X minutes.

Solution

extract vertices as geoms with ST_DumpPoints
compute difference between consecutive M values using LAG window function
query for the required difference

SELECT * FROM 
(select 
    ST_M(geom) - LAG( ST_M(geom)) OVER () diff, 
    path 
FROM ST_DumpPoints( 'LINESTRING M (0 0 0, 10 0 20, 12 0 40, 20 0 50, 21 0 70)')) p
WHERE diff > 10;

Query Invalid Zero-Length LineStrings

https://gis.stackexchange.com/questions/203125/st-intersects-with-degenerate-linestring?rq=1

Problem: Zero-length LineStrings are invalid, and hence cause spatial predicates such as ST_Intersects to return false.