The ST_ExteriorRing() function

The ST_ExteriorRing() function returns the exterior ring of a polygon as a linestring.

Syntax

ST_ExteriorRing(pl1 ST_Polygon)

Return type

ST_LineString

Example

An ornithologist studying the bird population on several South Sea islands knows that the feeding zone of the bird species of interest is restricted to the shoreline. As part of the calculation of the island's carrying capacity, the ornithologist requires the islands' perimeters. Some of the islands are so large, they have several ponds on them. However, the shorelines of the ponds are inhabited exclusively by another more aggressive bird species. Therefore, the ornithologist requires the perimeter of the exterior ring only of the islands.

The ID and name columns of the islands table identifies each island, while the land polygon column stores the island's geometry:

CREATE TABLE islands (id    integer,
                      name  varchar(32),
                      land  ST_Polygon);

The ST_ExteriorRing() function extracts the exterior ring of each island polygon as a linestring. The length of the linestring is calculated by the ST_Length() function. The linestring lengths are summarized by the SUM operator:

SELECT SUM(ST_Length(ST_ExteriorRing(land)))
  FROM islands;

As shown in the following figure, the exterior rings of the islands represent the ecological interface each island shares with the sea. Some of the islands have lakes, which are represented by the interior rings of the polygons.

This graphic is described in the surrounding text. — Figure 1: Islands and lakes