Select lines hanving their centroid inside a polygon, PostGIS

Question

How can I select a line having the centroid inside a polygon? there is a specific fuction?

Answer 1

You want a standard ST_Intersects filter, and then you have three options:

• use the ST_Centroid to get the (weighted) centroid of the lines:

  SELECT ln.*
  FROM   <lines> AS ln
  JOIN   <polygons> AS pl
    ON   ST_Intersects(pl.geom, ST_Centroid(ln.geom))
  ;
  

• use the ST_PointOnSurface to get a (non-weighted) center point of the lines

  SELECT ln.*
  FROM   <lines> AS ln
  JOIN   <polygons> AS pl
    ON   ST_Intersects(pl.geom, ST_PointOnSurface(ln.geom))
  ;

• use the interpolated midpoint (ST_LineInterpolatePoint) of the lines

  SELECT ln.*
  FROM   <lines> AS ln
  JOIN   <polygons> AS pl
    ON   ST_Intersects(pl.geom, ST_LineInterpolatePoint(ln.geom, 0.5))
  ;
  

Depending on your lines, all three may differ:

• ST_Centroid results in a point not guaranteed to be on the line (check the example in the docs) and is likely not what you want
• ST_PointOnSurface is extended in PostGIS to work on arbitrary geometries and guarantees^[*^] a point on the line around the geometric center using a deterministic algorithm
• ST_LineInterpolatePoint generates a point on the line at the given fraction of line length (0.5 for the midpoint) and guarantees^[*^] a point on the line

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^[*^] _{within the margin of floating point rounding errors}