PostGIS – How to Use ST_ConvexHull() Perpendicular to a Line

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I have a rather special requirement for a PostGIS geometry manipulation.

Consider the following case:

I have a polygon and a line feature. If I do a ST_ConvexHull() or ST_ConcaveHull() I would get something like on the left side. However, what I need is the area that encloses the polygon and the line where its perpendicular lines touch the polygon. Is there a function or another multiple step way to achieve this in PostGIS?

I think with ST_ShortestLine() from each vertex of the polygon to the line it may be possible. However, I still don't know, how I can get the area from the set of shortest lines.

Best Answer

OK. I figured it out myself. By doing a ST_ShortestLine() on each vertex of the polygon and then a ST_Convexhull() on this set of lines and a final ST_Difference() between that area and the area of the polygon I will get what I need.

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[GIS] PostGIS Dissolve/Union close polygons

I think you should use ST_Dump to disaggregate your Multi-part polygon created in your step (1):

SELECT ST_Dump( 
   ST_Union(ST_SnapToGrid(the_geom,0.0001))
   FROM parishes
   GROUP BY county_name
);

PostGIS – Raster Statistics for Polygon Areas in PostGIS

I've come up with a solution which works:

  SELECT x.bufid, (ST_SummaryStatsAgg(x.intersectx,1,true)).* 
  FROM
       (SELECT bufid, ST_Intersection(rast,1,ST_AsRaster(geom, rast),1) as intersectx
       FROM rasttable, buftable
       WHERE ST_Intersects(geom, rast)) as x
  WHERE x.intersectx IS NOT NULL
  GROUP BY bufid

I think the issue was with the geometry of the vector data (buftable) whereby there was no intersection occurring between it and the raster data for some rows (due to invalid geometry?).

The following part takes care of that issue:

WHERE x.intersectx IS NOT NULL

Best Answer

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PostGIS – Raster Statistics for Polygon Areas in PostGIS

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