Beschreibung

Berechnet ein POLYGON oder MULTIPOLYGON, das alle Punkte darstellt, deren Abstand zu einer Geometrie/Geografie kleiner oder gleich einem bestimmten Abstand ist. Ein negativer Abstand verkleinert die Geometrie, anstatt sie zu vergrößern. Ein negativer Abstand kann ein Polygon vollständig schrumpfen lassen; in diesem Fall wird POLYGON EMPTY zurückgegeben. Für Punkte und Linien geben negative Abstände immer leere Ergebnisse zurück.

Bei Geometrie wird die Entfernung in den Einheiten des räumlichen Bezugssystems der Geometrie angegeben. Für die Geografie wird die Entfernung in Metern angegeben.

Der optionale dritte Parameter steuert die Genauigkeit und den Stil des Puffers. Die Genauigkeit der Kreisbögen im Puffer wird als die Anzahl der Liniensegmente angegeben, die zur Annäherung an einen Viertelkreis verwendet werden (Standardwert ist 8). Der Pufferstil kann durch Angabe einer Liste von durch Leerzeichen getrennten Schlüssel=Wert-Paaren wie folgt spezifiziert werden:

Erweiterung: 2.5.0 - ST_Buffer ermöglicht jetzt auch eine seitliche Pufferzonenberechnung über side=both|left|right.

Verfügbarkeit: 1.5 - ST_Buffer wurde um die Unterstützung von Abschlusstücken/endcaps und Join-Typen erweitert. Diese können zum Beispiel dazu verwendet werden, um Linienzüge von Straßen in Straßenpolygone mit flachen oder rechtwinkeligen Abschlüssen anstatt mit runden Enden umzuwandeln. Ein schlanker Adapter für den geographischen Datentyp wurde hinzugefügt.

Wird vom GEOS Modul ausgeführt

Diese Methode implementiert die OGC Simple Features Implementation Specification for SQL 1.1. s2.1.1.3

Diese Methode setzt die SQL/MM-Spezifikation um. SQL-MM IEC 13249-3: 5.1.30

Beispiele

A buffered point approximates a circle. A buffered point forcing approximation of (see diagram) 2 points per quarter circle is poly with 8 sides (see diagram)

SELECT ST_NPoints(ST_Buffer(ST_GeomFromText('POINT(100 90)'), 50)) As promisingcircle_pcount,
ST_NPoints(ST_Buffer(ST_GeomFromText('POINT(100 90)'), 50, 2)) As lamecircle_pcount;

promisingcircle_pcount | lamecircle_pcount
------------------------+-------------------
             33 |                9

A lighter, less smooth circle using only 2 points per quarter circle is an octagon.

The following example creates a 100 meter octagon in NAD83 longitude/latitude by transforming to the Massachusetts state plane meter projection and then buffering.

SELECT ST_AsText(ST_Buffer(
ST_Transform(ST_SetSRID(ST_Point(-71.063526, 42.35785),4269), 26986),
100,2)) As octagon;

POLYGON((236057.5905746494 900908.7599186979,236028.30125276805 900838.0492405792,235957.5905746494 900808.7599186979,235886.87989653074 900838.0492405792,235857.5905746494 900908.7599186979,235886.87989653074 900979.4705968165,235957.5905746494 901008.7599186979,236028.30125276805 900979.4705968165,236057.5905746494 900908.7599186979))

Buffering a polygon by a positive distance and then by the same negative distance is a morphological closing operation. It can be useful for smoothing noisy boundaries, such as simplifying a coastline by removing narrow inlets and bays. The result intentionally changes the shape and area of the input, and the distance must be chosen in the units of the input SRS. For data in a geographic SRS, transform to a projected SRS suited to the operation first; for cartographic visualization in Web Mercator, using that display SRS can be appropriate.

-- Smooth a coastline using a 1 km closing operation, then simplify the result.
-- The example uses Massachusetts State Plane meter units for the operation.
WITH projected AS (
  SELECT id, ST_Transform(geom, 26986) AS geom
  FROM coast
),
closed AS (
  SELECT id, ST_Buffer(ST_Buffer(geom, 1000), -1000) AS geom
  FROM projected
)
SELECT id, ST_Transform(ST_SimplifyPreserveTopology(geom, 50), 4326) AS geom
FROM closed;

The same technique is described in Paul Ramsey's Removing Complexities. The final simplification step is optional; use ST_SimplifyPreserveTopology when polygonal validity matters, and check the result against the cartographic or analytical tolerance required by the application.

A small negative buffer can also be used as a heuristic to find narrow polygon spikes. The query below compares how much area and perimeter remain after erosion. A large erosion_index indicates that the perimeter dropped proportionally more than the area. This is only a preselection aid; the threshold and buffer distance must be chosen for the data units and the kind of spike being checked.

WITH sample(id, geom) AS (
  VALUES
    ('plain', 'POLYGON((0 0,10 0,10 10,0 10,0 0))'::geometry),
    ('spike', 'POLYGON((0 0,10 0,10 10,5.2 10,5 16,4.8 10,0 10,0 0))'::geometry)
),
eroded AS (
  SELECT id, geom, ST_Buffer(geom, -0.5) AS eroded_geom
  FROM sample
),
scored AS (
  SELECT id,
         round((
           ST_Area(eroded_geom) / ST_Area(geom) /
           NULLIF(ST_Perimeter(eroded_geom) / ST_Perimeter(geom), 0)
         )::numeric, 3) AS erosion_index
  FROM eroded
  WHERE NOT ST_IsEmpty(eroded_geom)
)
SELECT id, erosion_index, erosion_index > 1.1 AS possible_spike
FROM scored
ORDER BY id;

id   | erosion_index | possible_spike
-------+---------------+----------------
 plain |         0.900 | f
 spike |         1.147 | t

A variable-distance buffer can be approximated by buffering each vertex of a line by a distance interpolated along the line, building the convex hull for each consecutive pair of vertex buffers, and unioning the segment hulls. As with any geometry buffer, distances are in the units of the input SRS.

-- Taper a line buffer from 5 to 25 units along the line.
WITH line AS (
  SELECT 'LINESTRING(0 0, 100 0, 160 40)'::geometry AS geom
),
vertices AS (
  SELECT (dump).path[1] AS n,
         (dump).geom AS geom,
         line.geom AS line_geom
  FROM line
  CROSS JOIN LATERAL ST_DumpPoints(line.geom) AS dump
),
radii AS (
  SELECT n,
         geom,
         5 + (25 - 5) * ST_LineLocatePoint(line_geom, geom) AS radius
  FROM vertices
),
segments AS (
  SELECT n,
         geom,
         radius,
         lead(geom) OVER (ORDER BY n) AS next_geom,
         lead(radius) OVER (ORDER BY n) AS next_radius
  FROM radii
)
SELECT ST_Union(
         ST_ConvexHull(
           ST_Collect(
             ST_Buffer(geom, radius),
             ST_Buffer(next_geom, next_radius)
           )
         )
       ) AS tapered_buffer
FROM segments
WHERE next_geom IS NOT NULL;

Siehe auch

ST_Collect, ST_ConvexHull, ST_DWithin, ST_DumpPoints, ST_LineLocatePoint, ST_SetSRID, ST_SimplifyPreserveTopology, ST_Transform, ST_Union, ST_MakeValid