[OSGeo-UK] [External] QGIS query

[Al Graham]

Morning Barry

Thanks for looking at this and suggesting those possible routes forward.
I'll have a look at implementing the rule based styling later on. Sounds
like a useful thing to know in any case!

Cheers


On Thu, 21 Aug 2025, 17:40 Barry Rowlingson, <b.rowlingson at lancaster.ac.uk>
wrote:

> Actually, the previous approach (which bounced, oops) with a virtual layer
> is probably overkill, you can do some rule-based styling on the layer being
> input. Add a rule for $area < your_threshold and use a centroid style. Add
> an "else" rule for your desired style.  The red centroid dot appears on
> completion of small polygons.
>
> Not sure how to prevent creation of tiny polygons in the first place, but
> its only a Ctrl-Z away if you create one and its flagged as too small and
> you want to zap it.
>
> Barry
>
>
> On Thu, Aug 21, 2025 at 5:32 PM Barry Rowlingson <b.rowlingson at gmail.com>
> wrote:
>
>> I just tried creating a virtual layer based on the layer being digitised
>> with SQL like:
>>
>> select * from p where area(geometry) < 1000
>>
>> then when I create polygons with area < 1000 in my layer `p`, I get a
>> feature in the virtual layer. You could style this as a point at the
>> centroid. Then when you create a feature smaller than that threshold, you
>> get a red dot at its centroid.
>>
>> Seems to update instantly on completion of the polygon, you don't even
>> have to save the edited layer to get the virtual layer to update. At first
>> I wasn't seeing the virtual layer change but that was when I was styling
>> the virtual layer as polygons and they might have been hiding under the new
>> feature. Styled as centroid points they show up as I close the polygon.
>>
>>
>> Useful starting point?
>>
>> Barry
>>
>>
>>
>>
>>
>>
>>
>> On Thu, Aug 21, 2025 at 5:11 PM Al Graham via UK <uk at lists.osgeo.org>
>> wrote:
>>
>>> *This email originated outside the University. Check before clicking
>>> links or attachments.*
>>> Hi all
>>> A (hopefully) quick QGIS question for you.
>>>
>>> If I am digitising land cover from an aerial photo is there a way to
>>> limit the size of the polygon geometry to a minimum mappable unit (MMU)?
>>> For instance, say that size is 3 square metres, is it possible for QGIS to
>>> automatically flag, or reject, a new polygon that is 1m by 1m? Or is this
>>> something I would have to reconcile once the digitising has been completed?
>>>
>>> I think it's the latter option, but I thought I'd ask in case there's
>>> some special QGIS function I am unaware of.
>>>
>>> Cheers
>>> Al
>>>
>>
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