[Qgis-user] Line dissolve

[Nicolas Cadieux]

Hi,

I will look at the wkt output but it seems that with lines, dissolve will just create a single huge multiline.  The multi part to single part will just split it back up.  I suspect that dissolve only deals with common border in polygones and not line ends.  Will test tomorrow to figure out more details.

Nicolas Cadieux
https://gitlab.com/njacadieux

> Le 31 août 2021 à 03:57, DelazJ <delazj at gmail.com> a écrit :
> 
> 
> 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 Cadieux
>> https://gitlab.com/njacadieux
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