Calculating Convex Hull based on attributes in QGIS

Question

I have a layer with lots of (spatially grouped) lines - lines belonging to a group share the same value in a field. I would like to create convex hulls around each of these groups using QGIS 3.8.

I learned that Vector Geometry->Convex hull is not attribute specific and the batch functionality works with a set of layers, not specific on attributes.

I tried the model builder, but I'm struggling - I would need to first create a set of objects grouped by that attribute (in SQL it would be something like GROUP BY), and then something like a "do convex hull for each in set".

Or create a number of new layers based on the attribute and use a convex hull in batch mode? This would be ~100 new layers - could this be handled with QGIS?

score 9 · Accepted Answer · answered Aug 08 '19 at 08:14

9

There is another Convex Hull function in the "Vector geometry" -" "Minimum bounding geometry" tool. That one allows you to specify a field by with the geometries should be grouped.

answered Aug 08 '19 at 08:14

bugmenot123

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I didn't know this tool. It solves my problem and is able to not only produce convex hull but also bounding box etc. – roland Aug 08 '19 at 10:17

score 3 · Answer 2 · edited Nov 02 '23 at 11:56

3

Alternatively, since you are referencing SQL yourself, why not use the natively supported SpatiaLite dialect?

Get familiar with Virtual Layers and the DB Manager if not already, and run sth. like

SELECT <that_attribute>,
       ST_ConcaveHull(ST_Collect(geometry)) AS geometry
FROM   <layer_name>
GROUP BY
       <that_attribute>
;

edited Nov 02 '23 at 11:56

Taras

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answered Aug 08 '19 at 09:36

geozelot

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Caveat and benefit: Using this approach the hulls are going to be recalculated on each map interaction. – bugmenot123 Aug 08 '19 at 12:28
@bugmenot123 I don't see a caveat; you have the ultimate flexibility of spatial SQL: alter output with a simple change of functions or their parameters, no limit to what attributes/values you group by, and if you want persistent results (and only then, in contrast to the use of the built-in tools), just export to e.g. file – geozelot Aug 08 '19 at 13:30
It can become very slow. The user might not be aware that it is not a one-time operation but constantly recalculated. If their data is static, it is not really necessary. Don't get me wrong, this approach is awesome IF you know the caveats. – bugmenot123 Aug 08 '19 at 14:00
@bugmenot123 yeah I might be biased by the ways I do things ,) good to point that out! – geozelot Aug 13 '19 at 16:36

score 2 · Answer 3 · edited Nov 02 '23 at 11:56

2

You can create a convex hull based on an attribute using QGIS expressions with "Geometry Generator" or "Geometry by Expression" (see here for details about both options). Use this expression:

convex_hull(collect($geometry, group_by:="attribute_name"))

edited Nov 02 '23 at 11:56

Taras

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answered Nov 02 '23 at 00:31

Babel

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Calculating Convex Hull based on attributes in QGIS

3 Answers3