PostGIS Maximum Inscribed Circle

Question

I'm attempting to utilize the PostGIS function ST_MaximumInscribedCircle() but having some trouble interpreting the example. How can I feed the geometry into a from clause to retrieve the center and the radius values? I've tried sub-queries and CTE's but it doesn't seem to want to recognize it. Here is an example of what I've attempted:

SELECT radius, st_geomfromtext(ST_AsText(center)) AS center, st_geomfromtext(ST_AsText(nearest)) AS nearest
FROM (select ST_MaximumInscribedCircle(st_astext(st_union(s.geom))) from table s) as foo

This is the error I'm getting: SQL Error [42703]: ERROR: column "radius" does not exist

Answer 1

An elegant way to expand and access SETOF records (i.e. from Table Functions) is a LATERAL join:

SELECT
  mic.radius,
  mic.center,   --already a GEOMETRY(POINT) type
  mic.nearest   --already a GEOMETRY(POINT) type
FROM
  <table> AS t,
  LATERAL ST_MaximumInscribedCircle(t.geom) AS mic
;

If you require the base table geometries to get ST_Union'ed first, use a sub-query:

SELECT
  mic.radius,
  mic.center,   --already a GEOMETRY(POINT) type
  mic.nearest   --already a GEOMETRY(POINT) type
FROM
  (
    SELECT
      ST_Union(geom) AS geom
    FROM
      <table>
  ) AS ut,
  LATERAL ST_MaximumInscribedCircle(ut.geom) AS mic
;

---

Alternatively, you can use composite type access notation:

SELECT
  (ST_MaximumInscribedCircle(ST_Union(geom))).*
FROM
  <table>
;

to expand the record into its named fields, or:

SELECT
  (ut.mic).radius,
  (ut.mic).center,   --already a GEOMETRY(POINT) type
  (ut.mic).nearest   --already a GEOMETRY(POINT) type
FROM
  (
    SELECT
      ST_MaximumInscribedCircle(ST_Union(geom)) AS geom
    FROM
      <table>
  ) AS ut
;

or even:

SELECT
  (mic).radius,
  (mic).center,   --already a GEOMETRY(POINT) type
  (mic).nearest   --already a GEOMETRY(POINT) type
FROM
  ST_MaximumInscribedCircle((
    SELECT
      ST_Union(geom) AS geom
    FROM
      <table>
  )) AS mic
;

to get access to those fields by identifier.

Answer 2

If the example in https://postgis.net/docs/manual-3.2/ST_MaximumInscribedCircle.html works for you know that your PostGIS has the function and everything is fine so far.

SELECT radius, ST_AsText(center) AS center, ST_AsText(nearest) AS nearest
    FROM ST_MaximumInscribedCircle('POLYGON ((50 50, 150 50, 150 150, 50 150, 50 50))');

The synopsis of the function is

(geometry, geometry, double precision) ST_MaximumInscribedCircle(geometry geom);

You seem to use this as input

FROM (select ST_MaximumInscribedCircle(st_astext(st_union(s.geom)))

but st_astext returns a string and ST_MaximumInscribedCircle will fail. Leave st_astext out.

Then there is something that I cannot explain with the radius, center, and nearest. They cannot be used just by names as in the example when the geometry is selected from a table. Instead a syntax including the name of the geometry like in (radius(geom) must be used. I edited the query as follows and managed to get a result. My input table was "states" and the geometry column "wkb_geometry".

SELECT radius(geom), ST_AsText(center(geom)) AS center, ST_AsText(nearest(geom)) AS nearest from
(select (ST_MaximumInscribedCircle(st_union(s.wkb_geometry))) as geom  from states s) as foo;