[Qgis-user] Line dissolve

[DelazJ]

Hi,

> That does not work with lines unfortunately. I tried multiple times with
different combinations.  It works with polygons when common borders need to
be dissolved.

Nicolas, can you elaborate a bit more on the issues you have with the
"Dissolve + Multipart to singleparts" solution I provided few days ago. At
which step is it failing? The dissolve does not merge joining features into
a single one? I don't know the function internals but I'd expect it to work
this way and if not, would sound like a bug to report IMHO...

Regards,
Harrissou

Le mar. 31 août 2021 à 00:57, Nicolas Cadieux <njacadieux.gitlab at gmail.com>
a écrit :

> Thanks Charles,
>
> This will be very useful. Nice to know I'am not the only one going crazy
> with this!!!
>
> Nicolas
> On 2021-08-30 4:42 p.m., Charles Dixon-Paver wrote:
>
> I thought this would be easily solved with an expression but sadly I
> haven't been able to get reliable results at all.
>
> My first thought was
> collect($geometry,filter:=intersects($geometry,geometry(@parent))) and
> when that didn't work I tried various aggregates and union operations to no
> avail. Something like this might be useful if you can get it into a usable
> form (I managed to get various multigeometries in the form you are looking
> for but wasn't able to do much with them and there seemed to be artifacts):
>
>
> collect_geometries(array_foreach(aggregate(layer:='lines',aggregate:='array_agg',expression:=$geometry,filter:=intersects($geometry,geometry(@parent))), at element))
>
> I was actually waiting for someone else to come up with a more elegant
> solution, but it would probably also be possible to identify intersecting
> features and get the minimum/ maximum feature id for intersecting features
> with this:
>
> aggregate(layer:='lines',
> aggregate:='min',
> expression:="fid",
> filter:=intersects($geometry,geometry(@parent)))
>
> You could use that in the field calculator to create a field for
> "grouping" various intersecting features to dissolve on, however it's only
> going to identify the minimum of the currently intersecting features so you
> would need to iterate through the process multiple times. I can't think of
> an effective way to implement a "while" conditional either.
>
> It's a pretty clanky way to do it, but if you are really struggling I
> think it's possible.
>
> On Mon, 30 Aug 2021 at 22:10, Nicolas Cadieux <njacadieux.gitlab at gmail.com>
> wrote:
>
>> Hi,
>>
>> Seems like the plugin is available so I will try it out.  The ability to
>> create multi-linestring out of any touching lines (touch here means
>> intersects at the first or last node) would be nice to have it if I can
>> include it in a model.
>>
>> My goal is for this model to work for anyone with a basic install of QGIS
>> LTR or 3.20 with minimal efforts so I am trying to eliminate the need to
>> rely on external plugins or scripts.
>>
>> This is also my first attempt with models. It’s a learning curve but I am
>> having fun… I think this probably should have been a plugin but my
>> knowledge of GUI programming is basically 0.  That will be for a next
>> project…
>>
>> Nicolas Cadieux
>> https://gitlab.com/njacadieux
>>
>> Le 30 août 2021 à 14:59, David Strip <qgis-user at stripfamily.net> a
>> écrit :
>>
>>  I looked at the source for this plug-in and as far as I can tell  it
>> only merges lines that meet at endpoints. It ignores multi-lines, and only
>> creates linestrings, so there are no vertices of degree >2 (ie, no
>> junctions/forks). Also, the plug-in doesn't look for intersections between
>> vertices (since it only looks to merge endpoints.)
>> The StackExchange post appears to want multi-linestrings, which gives
>> forks and such. Is that what you need?
>>
>> On 8/30/2021 5:27 AM, Nicolas Cadieux wrote:
>>
>> Hi,
>> This is the plugin I was referring to.  I don’t see it in the current
>> plugins (experimental turned on)  so I figured it’s QGIS 2 only.
>>
>> https://github.com/ArMoraer/QGISMergeLines
>>
>> Nicolas Cadieux
>> https://gitlab.com/njacadieux
>>
>>
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> --
> Nicolas Cadieuxhttps://gitlab.com/njacadieux
>
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