ST_Buffer (Buffers / Proximity Shapes)

Use this skill when you need to create a polygon “buffer” around a geometry (point/line/polygon), typically for:

• proximity searches
• topology/contiguity validation
• “within X distance” visualization
• making thin lines easier to intersect (line → corridor)

Core rules

• Buffer units depend on the input type:
  • geometry buffers use the SRID’s coordinate units (degrees/meters/feet)
  • geography buffers use meters
• Do not buffer in EPSG:4326 degrees unless you intend degrees
• Prefer buffering in a projected CRS or using geography

Canonical patterns

Buffer in meters (recommended, easiest): geography

Good default when your geometry is in EPSG:4326 and you want meters.

SELECT
  ST_Buffer(geom::geography, 250)::geometry AS geom_buffer
FROM my_table;

Buffer in projected CRS (recommended for large jobs)

Use a projected SRID for consistent planar units.

SELECT
  ST_Transform(
    ST_Buffer(ST_Transform(geom, 3857), 250),
    4326
  ) AS geom_buffer_4326
FROM my_table;

Use your client/project SRID instead of 3857 when you have one.

Buffer and union (common validation pattern)

Useful for “contiguity” checks or merging segment corridors.

SELECT
  ST_Union(ST_Buffer(geom, 0.5)) AS buffered_union
FROM line_segments
WHERE id = $1;

Buffer for safer intersects (line corridor)

Turn a line into a corridor to catch near-misses.

SELECT
  ST_Buffer(line.geom, 0.5) AS line_corridor
FROM lines line;

Then intersect points against line_corridor.

Notes on negative buffers

Negative buffers shrink polygons (can produce empties):

SELECT ST_Buffer(poly.geom, -5)
FROM polygons poly;

Handle empty results if you use negative buffers.

Common mistakes

• Buffering in EPSG:4326 (degrees) when you intended meters/feet
• Assuming ST_Buffer(geometry, x) is meters (it is not)
• Not transforming to a projected SRID for planar work
• Buffering huge datasets without limits (expensive)

Summary

• Use geom::geography when you want meters
• Use projected SRIDs for consistent planar buffering
• Remember geometry units come from the SRID