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There are several answers for this question on GIS stackexchange but none using open/free software.

How can someone without access to the Esri suite remove interior holes from a polygon?

Example data is available here

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so i took the data given by Hugh_Kelley and i was able to find a good postgis solution.

I imported both shapefiles in PostgresSql and used this query.

I expected to find a lot of polygons with holes but it was mainly multi geometry as you can see on the picture.

CREATE TABLE public.result_union as (
SELECT St_SetSrid( ST_MakePolygon (St_ExteriorRing( St_union( (polys.geom)))),27700) AS poly_boundary
from polys)

With this query, I found the same geometry as the boundary shapefile given by Hugh.

enter image description here

The St_exteriorRing function only accept polygon, so you can not just St_Collect them.

You really need to do a St_Union to dissolve the boundary and if it gives back a MultiGeometrie which is possible, you will have to dump it with St_Dump and do a St_ExteriorRing on each part.

The performance here was ok , 13 secondes , but with the union involved, I suspect it can be hard to use this on a very high number of geometry.

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  • This is great, I think ST_Dumping the multipolygon and calling ST_ExteriorRing on each part was where I switched to QGIS. I'll apply it to the ~4500 polygons in the entirety of London and see how performance is. – Hugh_Kelley Aug 3 '19 at 12:13
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This may be possible to do in PostGIS using ST_Dump but when I tried I got an error saying:

Can't resolve struct type geometry_dump

A search for the error seemed to indicate that this meant that PostGIS wasn't installed correctly, but that seems unlikely in my case. I never resolved this error.

Instead I used QGIS, which is easily connected to a PostgreSQL database.

There is a Delete Holes function you can find by searching in the processing toolbox.

processing toolbox search

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    I would suggest you to look into Postgis Function and a combo of St_ExteriorRing with St_polygonize may do the trick. i'll try to make an answer with it later – Maximilien jaffrès Aug 1 '19 at 8:25
  • yeah I explored that combination yesterday actually, because it's supposed to do exactly that, but ST_ExteriorRing just returned NULL. Looking at the docs, maybe this was because the geometry was a multipolygon rather than a polygon. Then given the trouble converting to type polygon with ST_Dump a solution didn't seem likely. – Hugh_Kelley Aug 1 '19 at 8:53
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    polygon isn't a PostGIS type. It's the native (and moderately useless) geometry type. Your Answer is a Question in disguise. I suggest you rewrite the question to focus on this issue so it can be answered. – Vince Aug 1 '19 at 11:30
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    Can you share your data ? i will give it a shot if you can share it – Maximilien jaffrès Aug 1 '19 at 12:21
  • fair enough, QGIS works perfectly well and it's available to all but happy to help find a solution in postgis too. Added a link to a folder with the data. polys.shp is the raw data and boundary.shp is the required outcome. – Hugh_Kelley Aug 1 '19 at 13:07

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