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Suppose you have a polygon with an inner ring representing a hole, and inside that hole is another inner ring (an "island"):

The above shape can be generated using:

SELECT

  ST_MakePolygon(

    ST_ExteriorRing(ST_Buffer(center, 10, 10)),

    ARRAY[

      ST_ExteriorRing(ST_Buffer(center, 6, 6)),

      ST_ExteriorRing(ST_Buffer(center, 3, 3))

    ]

  ) AS geom

FROM ST_MakePoint(0, 0) AS center;

While this can be represented as a single polygon, I have a requirement to convert this into multiple polygons. I am working with a library that supports polygons with holes, but does not support "islands" inside those holes - thus, I need to be able to extract those islands into separate polygons. And in case those islands also have holes that have even more "inner" islands - I need to be able to be able to extract those inner islands as well, such that no polygon in the result set contains "islands". Multiple holes (inner rings) are allowed - just no islands inside any of those holes (no inner ring wholly contained inside another inner ring).

How can I achieve this using PostGIS?

Also just to expand my knowledge... is there a more precise terminology I can use instead of "island"? I'm aware of "inner ring", but an inner ring can represent either a hole, or an "island" inside a hole. Just wondering if there is a more specific term for when an inner ring represents an island vs a hole.

postgis

share

asked 6 mins ago

Sergey KSergey K

1484

add a comment | 

0

Suppose you have a polygon with an inner ring representing a hole, and inside that hole is another inner ring (an "island"):

The above shape can be generated using:

SELECT

  ST_MakePolygon(

    ST_ExteriorRing(ST_Buffer(center, 10, 10)),

    ARRAY[

      ST_ExteriorRing(ST_Buffer(center, 6, 6)),

      ST_ExteriorRing(ST_Buffer(center, 3, 3))

    ]

  ) AS geom

FROM ST_MakePoint(0, 0) AS center;

While this can be represented as a single polygon, I have a requirement to convert this into multiple polygons. I am working with a library that supports polygons with holes, but does not support "islands" inside those holes - thus, I need to be able to extract those islands into separate polygons. And in case those islands also have holes that have even more "inner" islands - I need to be able to be able to extract those inner islands as well, such that no polygon in the result set contains "islands". Multiple holes (inner rings) are allowed - just no islands inside any of those holes (no inner ring wholly contained inside another inner ring).

How can I achieve this using PostGIS?

Also just to expand my knowledge... is there a more precise terminology I can use instead of "island"? I'm aware of "inner ring", but an inner ring can represent either a hole, or an "island" inside a hole. Just wondering if there is a more specific term for when an inner ring represents an island vs a hole.

postgis

share

asked 6 mins ago

Sergey KSergey K

1484

add a comment | 

Suppose you have a polygon with an inner ring representing a hole, and inside that hole is another inner ring (an "island"):

The above shape can be generated using:

SELECT

  ST_MakePolygon(

    ST_ExteriorRing(ST_Buffer(center, 10, 10)),

    ARRAY[

      ST_ExteriorRing(ST_Buffer(center, 6, 6)),

      ST_ExteriorRing(ST_Buffer(center, 3, 3))

    ]

  ) AS geom

FROM ST_MakePoint(0, 0) AS center;

While this can be represented as a single polygon, I have a requirement to convert this into multiple polygons. I am working with a library that supports polygons with holes, but does not support "islands" inside those holes - thus, I need to be able to extract those islands into separate polygons. And in case those islands also have holes that have even more "inner" islands - I need to be able to be able to extract those inner islands as well, such that no polygon in the result set contains "islands". Multiple holes (inner rings) are allowed - just no islands inside any of those holes (no inner ring wholly contained inside another inner ring).

How can I achieve this using PostGIS?

Also just to expand my knowledge... is there a more precise terminology I can use instead of "island"? I'm aware of "inner ring", but an inner ring can represent either a hole, or an "island" inside a hole. Just wondering if there is a more specific term for when an inner ring represents an island vs a hole.

postgis

share

asked 6 mins ago

Sergey KSergey K

1484

SELECT

  ST_MakePolygon(

    ST_ExteriorRing(ST_Buffer(center, 10, 10)),

    ARRAY[

      ST_ExteriorRing(ST_Buffer(center, 6, 6)),

      ST_ExteriorRing(ST_Buffer(center, 3, 3))

    ]

  ) AS geom

FROM ST_MakePoint(0, 0) AS center;

share

asked 6 mins ago

Sergey KSergey K

1484

share

asked 6 mins ago

Sergey KSergey K

1484

asked 6 mins ago

Sergey KSergey K

1484

asked 6 mins ago

Sergey KSergey K

1484