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I have a map I generated that roughly estimates timber industry land ownership (designated by the green on the map) in the counties surrounding the Salish Sea in the US state of Washington:

map of timber industry lands in Salish Sea region

I want to remove parcels below a certain area threshold (e.g "only show parcels larger than 20 acres"). But the problem with this is that many of the larger timber tracts are broken up into a bunch of smaller adjacent real property parcels. Thus, a timber company might have a contiguous 20 acre parcel that is broken up into four 5-acre parcels, and a simple "features where area > 20 acres" search would miss this parcel.

So what I am trying to figure out how to do is show any parcels that are either (a) larger than 20 acres or (b) part of a connected cluster/group of parcels that collectively cover more than 20 acres.

For example, zooming in on part of the above map:

enter image description here

I want to filter out any parcels that are smaller than the reference parcel, unless they are part of a larger cluster of adjacent/connected parcels (such as the one I labeled with the blue arrows in the image), and generate a new layer with these smaller, disconnected parcels removed.

Is there an easy way to do this through QGIS?

If not, does anyone have any ideas for Python libraries or command-line utilities I could use to achieve this?

  • I see that someone downvoted this, and would appreciate guidance on how my question could be improved ... was it unclear what I'm asking? not enough information? bright colors annoying? – J. Taylor Jun 11 '18 at 10:33
  • People downvote for many reasons, and the criteria for downvoting are described in the downvote tooltip. I've seen people say that they will always/often downvote when they see the downvotes of others being questioned. – PolyGeo Jun 11 '18 at 23:59
  • PolyGeo - I'm aware of what the criteria for downvoting are, but don't see how my question meets any of them ... Either way, I don't really care if people downvote me for asking, because I'm not trying to win points (just trying to figure out how to do the thing I described above). I'll just ask on the QGIS mailing list ¯_(ツ)_/¯ – J. Taylor Jun 12 '18 at 2:57
  • One thing I see is that it asks two questions, although that is more often a reason for a close vote as too broad. Someone without rep to close vote might downvote as an alternative and consider too broad for focused Q&A as being not useful. – PolyGeo Jun 12 '18 at 3:05
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You could use GRASS' v.dissolve on a common field value, filter out the smaller areas, and then use select by location from the vector menu to find which of your original parcels overlap the filtered areas.

  • Ah thanks! The term "dissolve" is exactly what I needed. I used GRASS v.dissolve in Processing toolbox, as you suggested. Then I just had to filter out all features of the generated layer less than N acres. Appreciate the help :) – J. Taylor Jun 13 '18 at 1:20

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