PostGIS – Creating One-Sided Buffers or Parallel Lines

bufferparallel linespolyline-creationpostgis

I'm looking for a way to create so-called one-sided buffers or parallel lines in PostGIS. Example: 1. buffer, 2. one-side, 3. both-sides

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I've found some discussion on the mailing list from 2009 and information that it has been implemented in GEOS, but nothing about the current PostGIS status.

In case the function is not implemented yet, do you know any workarounds? Is it possible to cut off one side of a normal buffer?

Best Answer

Proper one-sided buffers were supposed to have landed in 1.5, but it looks to me that while the styles did land, sidedness didn't make it in. There is however a current patchset which exposes GEOSSingleSidedBuffer and performs the one-sided buffer as expected, under the name ST_OffsetCurve; see further background in ticket #413. In use:

select ST_AsText(ST_OffsetCurve(
ST_GeomFromText('LINESTRING(10 10,10 20, 20 20 )'),
1,'right', 'join=mitre mitre_limit=5.0'));
--------------
LINESTRING(20 19,11 19,11 10)

Related Solutions

QGIS – Creating Dissolved Buffers Using PostGIS

ST_Collect is probably not the function you are looking for, as this simply combines geometries into a geometry collection of some type, and does not actually union/dissolve them. ST_Union, on the other hand, does dissolve overlaps, and assuming polygonal input (which is most probable in conjunction with ST_Buffer and an input table called point), and overlapping polygons will return a MultiPolygon. To return the individual unioned (dissolved) polygons you need to use ST_Dump which returns the geometries and a path id, which can be used as the gid required for visualization in QGIS -- as it is an array, you need to use [1] to access it. You will also want to union the buffers, rather than the other way round, as you currently have. Putting this altogether, you get:

CREATE VIEW buffer40units AS
SELECT 
   g.path[1] as gid, 
   g.geom::geometry(Polygon, 31492) as geom 
FROM
   (SELECT 
     (ST_Dump(ST_UNION(ST_Buffer(geom, 40)))).* 
   FROM point
) as g;

[GIS] How to select parallel lines in postgis

It would look something like this.

SELECT l1.geom, l2.geom AS non_intersecting_line
FROM lines AS l1
JOIN lines AS l2
  ON (
    ST_Intersects( ST_Buffer(l1.geom, 50), l2.geom )
    AND NOT ST_Intersects(l1.geom, l2.geom)
  )
WHERE l1.priority = 6;

WARNING: This doesn't strictly find parallel "lines." PostGIS is real world and not an abstract modeling tool. It doesn't have a notion of y=mx+b to constitute "parallel". This finds linesegments that do not intersect within 50 meters. They can be visually perpendicular non-intersecting lines in the United States and Zimbabwe, and this query will return them.

If you need visually parallel line segments defined by two points

Calculate the azimuth of the desired line defined by two points.
Calculate the azimuth of the line for comparison.
Permit a margin of error for rounding points.

This still won't find lines near each other, but they should be parallel. For instance, lines of longitude are parallel with other lines of longitude but they can cross earth as separate points on the geode.

If you need visually parallel lines defined by more than two points: tough. You can either calculate the angle with any two points or you're not talking about a line anyway.

Update with sample data

This is exactly the problem with your description..

Green is the buffer zone 50 meters.

CREATE TABLE service_rds_buffer AS
SELECT gid, name, priority, ST_Buffer(geom, 50)
FROM service_rds WHERE priority = 6;

The two lines inside the green are the only two roads where priority=6
That road curves. It's not a line. It's a line segment that represents a road composed of 121 discrete points. What does it mean to be parallel to a line segment like that? It means nothing. Which takes you back to my original query.

Best Answer

Related Solutions

QGIS – Creating Dissolved Buffers Using PostGIS

[GIS] How to select parallel lines in postgis

Update with sample data

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