postgresqlgispostgis

Create Postgis Polygon from Geometry Points with adding X metres to polygon boundaries

I need to create a polygon that surrounds a given set of points using PostGIS.
Additionally, the polygon should extend slightly beyond the boundary points by a specified distance (ideally in meters), making the resulting shape slightly larger.

Could someone advise if this is possible and how it can be achieved with PostGIS?

Solution

  • You can ST_Collect() your geometries, pass that to ST_ConvexHull() and take an ST_Buffer() around that:
    demo at db<>fiddle

    select st_buffer(st_convexhull(st_collect(geom)),9)
from your_table
group by cluster_num;
    st_astext
    POLYGON((200 191,191 200,191 300,200 309,300 309,309 300,309 200,300 191,200 191))
    POLYGON((6.363961030678928 -6.363961030678928,-3.444150891285813 -8.314915792601578,-9 0,-9 100,0 109,100 109,108.31491579260158 103.4441508912858,106.36396103067892 93.63603896932108,6.363961030678928 -6.363961030678928))

    enter image description here

    Note that the buffer size 9 is measured in your geom column unit dictated by the SRID if it's a geometry, meters if it's a geography. Popular SRID:4326 is in degrees, so you should either cast that to ::geography or ST_Transform() to a meter-based local coordinate reference system.
    There are also SRIDs in feet and nautical miles, so the length of that 9 changes accordingly.

    If you're also looking for a way to cluster them to decide which geometries should be together, there's ST_ClusterWithinWin().

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