[postgis-users] generate a geometry column of random point, line and polygon

[Imre Samu]

related:
Random Geometry Generation with PostGIS ( by Paul Ramsey, Sep 11, 2023 )
https://www.crunchydata.com/blog/random-geometry-generation-with-postgis

Maybe we could add a few new "ST_generate*" functions to PostGIS:
- ST_GenerateLine(n_vertices INTEGER, bounding_geometry GEOMETRY)
- ST_GeneratePolygons(n_vertices INTEGER, bounding_geometry GEOMETRY)

Imre

Regina Obe <lr at pcorp.us> ezt írta (időpont: 2023. szept. 11., H, 10:09):

> You need to do
>
>
>
> CREATE EXTENSION postgis_sfcgal;
>
>
>
> It’s not part of the postgis extension.  If you don’t have that extension,
> then you can’t use this function.
>
>
>
>
>
> You next best bet is using  ST_ConcaveHull
>
>
>
> https://postgis.net/docs/ST_ConcaveHull.html
>
>
>
>
>
> *From:* postgis-users <postgis-users-bounces at lists.osgeo.org> *On Behalf
> Of *Shaozhong SHI
> *Sent:* Monday, September 11, 2023 3:36 AM
> *To:* PostGIS Users Discussion <postgis-users at lists.osgeo.org>
> *Subject:* Re: [postgis-users] generate a geometry column of random
> point, line and polygon
>
>
>
> Hi, Regina,
>
>
>
> It did not work.
>
> ERROR: function st_optimalalphashape(geometry) does not exist LINE 1:
> SELECT min(gs) , max(gs), st_optimalalphashape(st_collect(S... ^ HINT: No
> function matches the given name and argument types. You might need to add
> explicit type casts. SQL state: 42883 Character: 28
>
>
>
> On Mon, 11 Sept 2023 at 08:31, Regina Obe <lr at pcorp.us> wrote:
>
> You are using the wrong version of ST_Collect.  You want to use the
> aggregate form.  You are using the two point form  -
> https://postgis.net/docs/en/ST_Collect.html
>
>
>
> Also when you aggregate, you can’t include the gs in there.
>
>
>
> Try:
>
>
>
> SELECT  min(gs) , max(gs),
> st_optimalalphashape(st_collect(ST_MakePoint(random()*10,random()*10)) )
>
> from generate_series(1,100) gs;
>
>
>
> *From:* postgis-users <postgis-users-bounces at lists.osgeo.org> *On Behalf
> Of *Shaozhong SHI
> *Sent:* Monday, September 11, 2023 3:21 AM
> *To:* PostGIS Users Discussion <postgis-users at lists.osgeo.org>
> *Subject:* Re: [postgis-users] generate a geometry column of random
> point, line and polygon
>
>
>
> I tested St_optimalphashape and it does not work.
>
>
>
> select gs,
> st_optimalalphashape(st_collect(ST_MakePoint(random()*10,random()*10),ST_MakePoint(random()*10,random()*10))::geometry)
>
> from generate_series(1,100) gs;
>
>
>
> Example given here does not work either.
>
> ST_OptimalAlphaShape (postgis.net)
> <https://postgis.net/docs/en/ST_OptimalAlphaShape.html>
>
>
>
> Regards,
>
>
>
> David
>
>
>
> On Fri, 8 Sept 2023 at 16:16, Regina Obe <lr at pcorp.us> wrote:
>
> David,
>
>
>
> The example query in the docs, is as simple as it gets.  It’s a
> self-contained example you can just run, but it does return a multipoint
> and I realize now the docs don’t make it clear ST_GeneratePoints returns a
> single geometry that is a multipoint.
>
>
>
> IF you want individual points, you’d combine with ST_DumpPoints.
>
> I’ll add such an example to the docs.
>
>
>
> So here is an example you can apply to a table of polygons
>
>
>
> For polygons you can do something like below where p is the table name and
> geom is the polygon column.
>
> This will generate 100 random points for each polygon
>
>
>
>
>
> SELECT p.id, dp.path[1], dp.geom
>
> FROM p, ST_DumpPoints(ST_GeneratePoints(p.geom, 100)) AS dp;
>
>
>
> The p.id and path I just threw in cause I find them useful, but you could
> leave them out.
>
>
>
>
>
> Here is a self-contained using the example table in docs:
>
>
>
> WITH p AS (  SELECT 1 AS id, ST_Buffer(
>
>         ST_GeomFromText(
>
>         'LINESTRING(50 50,150 150,150 50)'),
>
>         10, 'endcap=round join=round') AS geom)
>
> SELECT p.id, dp.path[1], dp.geom
>
> FROM p, ST_DumpPoints(ST_GeneratePoints(p.geom, 100)) AS dp;
>
>
>
>
>
> *From:* postgis-users <postgis-users-bounces at lists.osgeo.org> *On Behalf
> Of *Shaozhong SHI
> *Sent:* Friday, September 8, 2023 10:54 AM
> *To:* PostGIS Users Discussion <postgis-users at lists.osgeo.org>
> *Subject:* Re: [postgis-users] generate a geometry column of random
> point, line and polygon
>
>
>
> The simplicity is beautiful.  Any simple script to do so?  I think that
> will be an useful addition.
>
> Regards,
>
> David
>
> On Friday, 8 September 2023, Regina Obe <lr at pcorp.us> wrote:
>
> David,
>
>
>
> For this are you needing to generate random points from a line or polygon
> or are you trying to generate random polygons, lines, and points?
>
>
>
> If you need to generate random points from a polygon:
>
>
>
> Use ST_GeneratePoints:  https://postgis.net/docs/en/ST_GeneratePoints.html
>
> Note there is a option seed argument, that will give you the same exact
> answer if you give it the same seed, but without that the generated points
> will be different each time.
>
>
>
> As I recall, ST_GeneratePoints only works with areals so won’t work with a
> line, however you can buffer a line very thinly to do the same. Use a flat
> buffer:
>
>
>
> https://postgis.net/docs/en/ST_Buffer.html
>
>
>
> SELECT ST_GeneratePoints(ST_Buffer(
>
> ST_GeomFromText(
>
>   'LINESTRING(50 50,150 150,150 50)'
>
> ), 0.5, 'endcap=square join=round'), 1000);
>
>
>
>
>
> If you want to generate random polygons, you could use ST_ConcaveHull or
> ST_AlphaShape around the section of a polygon you did a ST_GeneratePoints on
>
>
>
> And then use something like https://postgis.net/docs/en/ST_Subdivide.html
> to chop up the polygons.
>
>
>
> To get a linestring out of that (It will be closed), you can take the
> boundary of any of the above
>
>
>
> https://postgis.net/docs/ST_Boundary.html
>
>
>
> Hope that helps,
>
> Regina
>
>
>
> *From:* postgis-users <postgis-users-bounces at lists.osgeo.org> *On Behalf
> Of *Shaozhong SHI
> *Sent:* Friday, September 8, 2023 7:56 AM
> *To:* PostGIS Users Discussion <postgis-users at lists.osgeo.org>
> *Subject:* [postgis-users] generate a geometry column of random point,
> line and polygon
>
>
>
> Is a simple way to do this?
>
>
>
> Regards,
>
>
>
> David
>
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