[GIS] Trying to combine nearby points into a polygon

intersectionjts-topology-suitepostgisspatial-indexunion

We have collected millions of cell phone strength readings. I am trying to group points of the same strenght that are within a distance of each other in to a polygon. I asked on the
postgis mailing list but didn't get a response.

To make the process simpler, I already extracted all the point of the same strength to one table. So I have a table of 1.5 million rows. It has a gist index on the geom field and btree indexes on the uniqueid of each row and on the 'clusterid' which is supposed to be the same for all points that are in the same 'cluster' (ie point a which is within distance of point c which is within distance of point z would all be the same cluster).

How would you accomplish this task???

I did wonder whether it might be worth trying to load the geometries in to memory and use JTS to process them? Although I doubt that would be any better.

Thanks – Bryan.

Best Answer

I would not use a conventional point->polygon process because that expects your points to define the boundary of a polygon and it doesn't sound like yours do. It sounds like yours are hotspots that are somehow related.

However, there are lots of ways to create polygons for this sort of situation depending on what is sensitive in your data. Here's a few quick ideas (one or more of which might be appropriate to your use-case):

  • Buffer your points by your cluster distance and then dissolve the buffers based on the clusterid
  • Create a convex hull of your points, one per clusterid
  • Convert the points to a raster with a resolution of half your distance. Perhaps using the hexagonal cell raster available in QGIS would be good, you can the convert the raster to polygons
  • Interpolate a raster from the points using a finer resolution than the option above, but your interpolation method will need to be chosen based on what is appropriate to your use case.
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