ST_ExteriorRing

What is ST_ExteriorRing?

ST_ExteriorRing is a PostGIS function that returns the outer shell of a POLYGON as a closed LINESTRING. It does not include interior ring (hole) geometry.

ST_ExteriorRing(geometry polygon) → geometry

Only POLYGON input is supported — MULTIPOLYGON or other types return NULL. SRID and dimensionality are preserved; the result is always closed (first equals last vertex).

When would you use ST_ExteriorRing?

Use ST_ExteriorRing to extract just the outer boundary of a polygon — for example when drawing outlines, computing perimeter ignoring holes, or using the ring as input to ST_MakeLine edits:

1SELECT id,
2       ST_Length(ST_ExteriorRing(geom)) AS outer_perimeter_m
3FROM parcels
4WHERE GeometryType(geom) = 'POLYGON';

For polygons with holes, ST_ExteriorRing discards them; use ST_Boundary to get every ring (exterior plus interior) combined.

FAQs

How is ST_ExteriorRing different from ST_Boundary?

ST_ExteriorRing returns only the outer ring as a single LINESTRING. ST_Boundary returns all rings (exterior plus every hole) as a MULTILINESTRING when holes exist.

Can ST_ExteriorRing handle MultiPolygons?

No — it returns NULL on MULTIPOLYGON. Unpack first with ST_Dump or select individual components with ST_GeometryN.

Is the output a closed LineString?

Yes. The first and last vertices coincide. Use ST_IsClosed or ST_IsRing to verify.

What SRID does the output have?

Same as the input polygon. Dimensionality (Z, M) is preserved at each ring vertex.